Sights around Lake Washington
It promised to be a beautiful day, so we decided to take a look at Lake Washington, just east of Seattle. This elongate water body was carved by glaciers between 19 and 16 thousand years ago. It was part of Puget Sound until about 5700 years ago, when river sediment isolated it from the sea, allowing it to become fresh water. The original outlet at the southern end wasn’t good enough for early American settlers, so they got approval from Congress to construct a canal with locks to connect Lake Washington to Puget Sound, a drop of 20 feet in elevation. As fans of technological progress, we wanted to see this for ourselves.
The Army Corps of Engineers began construction of a set of locks in 1911, under the supervision of Hiram M. Chittenden. As the sign proclaims, his name remains associated with them; however, they are more commonly known as the Ballard Locks after the city where they were built. The site includes a number of stone-faced buildings and has since come to include a botanical garden.
There are two locks, one for larger, commercial vessels (on the other side of the control building) and a smaller lock. The larger one has two sets of gates to accommodate barges.
Several private boats used the lock while we were there. It takes maybe 15 minutes for the water level to equilibrate whether going upstream or downstream (towards Puget Sound).
The spillway was partially open and the turbulent flow created a very dangerous scene. I wouldn’t want to fall into that water.
Salmon have historically swam into Lake Washington to spawn through the small river (Black River) that drained it before the ship canal was constructed. That outlet is apparently filled with sediment now and the area completely covered with development, including Boeing Aircraft’s main plant, where the Museum of Flight is located. The lock design includes a series of pools called a fish ladder that allows the salmon to get to their spawning areas. I guess they figured out the new route. There’s a viewing room to the right where the fish can be seen making their way up the ladder, but there were no fish on this day; and I forgot to take a picture.
The roof of the fish ladder observation room is decorated with this unlabeled artwork.
Just a short drive from Ballard Locks is Washington Park, which includes an arboretum and Japanese Garden. We took a long walk around the park but didn’t make it to the Garden, which has its own parking area.
The walking paths go out to an island, where State Route 520 crosses Lake Washington, partly using a unique floating bridge construction. Traffic was pretty loud out there. I wouldn’t enjoy having lunch on one of the picnic tables.
These are typical plantings along the paths in the arboretum. I don’t know anything about plants, so I asked CoPilot (giving it the location): the low, flowering shrub is probably Pieris japonica (aka Lily of the Valley shrub); and the tree may be Stewartia.
There were numerous masses of flowering shrubs like this, which is probably a dwarf Rhododendron (according to CoPilot). There were a lot of plants within a group, which didn’t look anything like each other; however, the signs explained that the arboretum had large collections of Rhododendrons and Magnolias, for example.
It was a beautiful spring day, and Lake Washington was the perfect place to spend it. I won’t post a photo of the excellent, home-made hamburger and cold beer I enjoyed at Skillet and Vine after a tiring morning of basking in a warm sun.
Mount St Helens After Forty-Five Years
Introduction
I was a geology student at Arizona State University in May, 1980, when Mount St. Helens made the headlines. It is the largest volcanic eruption in North America, and when one-cubic-mile of mountain collapsed, it became, and remains, the largest landslide in human history. I followed the progress of geological investigation into the eruption with interest as I pursued my education, but progress was slow. It isn’t easy to reconstruct an event that occurred in a few minutes. The area was too dangerous to approach for more than a year because of gas explosions from within the pile of debris, which reached 600 feet in thickness.
Mount St. Helens faded from memory for decades, eventually becoming just another geologic event in a long chain of cataclysms covering billions of years. I never thought about it until I found myself living less than 100 miles from ground zero. I had to check it out. This post is a brief summary of what I found when I visited Mount St. Helens National Volcanic Monument. I hope I can convey some of the excitement I felt at stepping on ground that was literally on fire less than fifty years ago.
Figure 1. I arrived at about 10:30 in the morning and got this image before low clouds settled in, accompanied by fog later in the day. The characteristic volcanic cone is missing; the top of the mountain slants slightly upward to the right in the center of this image. That isn’t a lake in front of St. Helens, just fog collecting in the valley that feeds the Toutle River.
Figure 2. Mount St. Helens is about 2.5 hours from Tacoma. The closest you can drive is Hummocks Trail, but a four-mile hike will take you to the rim of the caldera. Maybe another time.
Older Volcaniclastic Rocks
Figure 3. Mount St. Helens is less than forty-thousand years old, but it is constructed on a thick sequence (~2 miles thick) of volcanic rocks as old as 300-thousand years. Beneath these Pleistocene volcaniclastic rocks mixed rocks of uncertain age that comprise “bed rock” in this area; however, these igneous and sedimentary rocks are much older–spanning the Oligocene epoch (~34-23 Ma). I stopped to look at several road cuts along the new Highway 504 (the original is buried under debris).
A. Exposure of andesite volcanic rocks showing a complex eruption history, which includes ash layers and what looks like tuff (ash so hot it melted together to form a glass-like volcanic rock).
B. Close-up from the left side of (A) showing a layer of volcanic breccia that is now vertical. Individual clasts are visible but there is very little matrix. The dashed line indicates the approximate bedding plane of the layer. The rounded block labeled with ?? is about 12 feet in diameter. This is a puzzling structure. My guess is that there was a collapse of one or more volcaniclastic layers into a ravine after deposition. A jumble of material.
C. Photo of a complicated structure separating the left side of the exposure (tilted beds and blocks) from the right side (horizontal ash layers). The dash line is meant as a reference to the vertical beds in (B), but it was difficult to determine orientation. However, a clear change in texture suggests a depression, which may have been a conduit for volcanic material to erupt. The BLOCK/PLUG label reflects this interpretation. Within this “BLOCK” there is a discrete region of thin, irregular bedding that I’ve labeled (for convenience) as a CHANNEL? The question mark reflects my doubts about this identifier. Nevertheless, this road cut exposes a sequence of events: lava and ash being erupted onto an irregular volcanic landscape; probable surface erosion for some period of time; physical disruption and collapse, probably while still hot, of some part of later volcanic material. I don’t have enough experience with volcaniclastic sediments to say anymore than that.
D. The pushpin shows the location of this road cut. The blue ribbon is Coldwater Lake, where Hummocks Trail is located (see Fig. 2); this exposure is more than ten miles from the caldera, and hundreds of feet above the valley floor.
Figure 4. This post-eruption road cut is a mile or more further from the caldera. The solid line was a striking lineation that could be a fracture or possibly a contact between eruption beds. The dash-dot lines are apparent bedding planes between andesite flows. There is no ash present at this location. The dash lines delineate what I’m calling a BRECCIA because there is no evidence of bedding and the overall appearance is irregular; also, some large blocks were evident, although they could be a result of differential weathering. Lava flows are notoriously difficult to trace any distance laterally. Nevertheless, this exposure is similar in appearance to Fig. 3 with respect to the discontinuity between identifiable volcaniclastic deposits.
Figure 5. This road cut is located (see inset map) in an area the geologic map (Rock D) identifies as volcaniclastic rocks of Oligocene age (33.9-23.04 Ma). This is a smaller exposure than seen in Figs. 3 and 4, but it is also very different in appearance. That could be an optical illusion; close examination of the photo (I took it from my vehicle stopped in the middle of the road) suggests to me that this is ash that was so hot it formed what is called a welded tuff when it fell, after being blown into the atmosphere by an eruption. The light color suggests a magmatic composition more like granite than gabbro, or even diorite. That wouldn’t be surprising in the complex magmatic environment of a subduction zone, where partially melted, oceanic crust (e.g. gabbro and basalt) chemically mixes with continental crust (e.g. granite and diorite).
This completes my survey of older volcanic rocks near Mount St. Helens. There are no rocks exposed that explain the apparent hiatus in volcanism between about 23 and 2 million years ago. That is a story for another day…
Volcaniclastic Deposits from May 1980 Eruption
Figure 6. Photo A was taken a few days before the eruption in May, 1980. Image B was taken a few days afterward. The pre-eruption volcano was 9677 feet tall, but the obliterated peak is only 8365 feet. The total volume of material displaced exceeded one cubic mile, most of which was rock that collapsed during the rock slide preceding the actual eruption. The flat area fronting the volcano in (B) is a large fraction of the previous peak, which filled in several channels originating at the volcano. Note the holes in (B), which are probably blow-outs of gases trapped in the debris.
Figure 7. (A) This plaque was located along Hummocks Trail. I’ve supplemented it with the map of volcaniclastic deposits shown in (B). I will focus on the three major volcaniclastic deposits I encountered at Hummocks Trail (blue circle in B), which are numbered 1-3 in both figures.
The first stage was collapse of the north flank of Mount St. Helens (brown in A and cross-hatched in B). This was a run-of-the-mill massive landslide that followed existing drainage, until stage two occurred.
When the rock containing the highly pressurized magma was removed. The resulting release of pressure created an explosion equivalent to 10-50 megatons of TNT; although a volcanic eruption is not analogous to a nuclear explosion, the energy released was roughly 1600 times the energy of the Hiroshima atomic bomb. It moved a lot of rock. This blast occurred seconds after stage 1 began; the hot gases overtook the rock slide, driving rock and debris up the sides of the valley, removing everything, including top soil, within a few miles of the volcano. The orange area in B shows how widespread this explosion was.
Stage 3 was a pyroclastic flow (red in B), which was limited to the immediate vicinity of the caldera; however, what goes up must come down, so several inches of ash were deposited as far as Spokane, Washington–400 miles distant.
Video 1. This video shows debris from the landslide (Stage 1) that was pushed at least 300 feet from the original valley floor to the ridge by the unimaginable blast of Stage 2. The main slide followed Toutle River (North Fork), but with this impetus, debris was launched over the canyon walls. That’s what this video shows. It’s like the heavy stuff collected at the top of the ridge whereas the sand/gravel-sized debris was blown miles further.
Figure 8. This photo is looking south towards Mount St. Helens from Hummocks Trail (see Figs. 2 and 7B for location) along the Toutle River (North Fork), which is more than a hundred feet lower, even after being filled with debris from the 1980 eruption. Note the difference between this scene and the deposit from Video 1. It’s difficult to comprehend the dynamics of a blast (Stage 2) capable of pushing rock (Stage 1) up this slope; when the explosion lost energy it simply dumped its load, creating a surreal landscape. The debris has very little clay and is thus non cohesive; thus slumps like this are common. Note the small hill and “ridge” in the middle of the photo. This terrain makes no sense, other than erosion has been etching it for 40 years.
Figure 9. This layer of ash I encountered along the Hummocks Trail is a remnant of a vast sheet originally deposited during Stages 2/3–breaking such a continuous eruption sequence into stages is useful but not particularly elucidating. It all happened too fast to comprehend. You might call this an “over-bank” deposit, like when a river tops its natural levee and deposits fine-grained sediment on its floodplain.
Figure 10. Hummocks Trail followed a small stream through a bizarre landscape comprising multiple small ponds like this one. A dam constructed of eruption debris (left-to-right in the center of the photo) has blocked the stream flow, creating a meandering channel during low-water conditions; but I can imagine all those dormant grasses erupting later this spring. I encountered several of these marshes, each one defined by water trying to find a way out of this labyrinth. According to Fig. 2, this is the outflow from Coldwater lake; the eruption partially blocked its path but nature is finding a way through this discombobulated landscape.
Figure 11. This photo perfectly captures the topography of Hummocks Trail. This is the northern end of the trail, which is a little higher; the Toutle River (North Fork) is more distant and the cliffs we saw in Fig. 8 have receded, replaced by moderate slope; however, note the fine-scale nature of the hummocks here. They are ubiquitous but contain only a few scattered boulders.
Figure 12. This is a view of Coldwater Lake from the visitor center, looking south towards Mount St. Helens. There are two things to note from this image: 1) the Toutle River valley is a broad, flat plain because it was filled with debris from Stage1; 2) the overflow deposits (e.g. Video 1) have blocked the lake and raised its level. The source of water we saw in Fig. 10 is evident in the center of the photo. This isn’t a sedimentary dam, created by flood deposits, or even a glacial dam produced from the debris scraped up by thick ice sheets. The shoreline was modified when Mount St. Helens collapsed and then exploded with unbelievable violence.
Final Thoughts
The eruption of Mount St. Helens in 1980 gave geologists a rare opportunity to see how Earth produces sausage. Let me explain my metaphor.
Ocean crust is denser than continental crust it encounters at a convergent plate boundary, and at a lower elevation. It is thus overridden by the continent, forcing it to dive into denser and hotter rocks at depths of hundreds of miles. This obviously displaces hot and ductile rocks, heating the subducting rocks even more; the sausage-making operation has begun. Still pushed from behind by the conveyor belt of oceanic crust being subducted, this basalt/gabbro mixture heats enough to boil off the water contained in its sedimentary cover as well as any constituents with a reduced melting point (e.g. silicon-rich minerals). This superheated material rises through tens-to-hundreds of miles of crust, heating it and producing a mixture of oceanic and continental crust.
Plate tectonics keeps turning the crank on the sausage machine for tens (even hundreds) of millions of years. The molten mass of magma keeps rising because it is less dense than the rocks through which it is passing, until the pressure has decreased enough for it to find cracks in the solid crust. The magma fills these cracks, which often lead to fissures in the surface; the magma erupts. The magma cools and begins to solidify, forming a pluton at depths of several miles to tens of miles.
This process keeps operating, in fits and starts, until the tectonic plates change direction. In the meantime, the upper crust has been filled with multiple plutons (solidified magma chambers), sometimes squeezed into the volcanic rocks from previous eruptions. Individual magma chambers/plutons are about one mile in diameter, although they take many shapes, and remain active for less than one-hundred-thousand years (often much less). These are injections of fluid rising through weak points in the crust, driven by pressure paths–not missiles launched from the mantle. They are like the sweat on your brow after a hard workout.
Mount St. Helens has released a lot of pressure, although it is still very dangerous at human scales. Chances are that this magma chamber will solidify within the next hundred-thousand years; but the unstoppable sausage machine will continue cranking out more mixed material in a molten form until it has filled every nook and cranny in the upper crust. That’s how mountain ranges like the Sierra Nevada were formed. It takes a really long time.
Mount St Helens probably won’t erupt again at the scale of the 1980 event, but that doesn’t mean much if you happen to be camping near the caldera when a smaller eruption occurs. After all, the magma chamber is still extruding lava in an unsteady process that is currently pretty slow. However, we humans can’t see fractures in bed rock; thus, the next and the next, etc, magma chamber could appear anywhere from Northern California to British Columbia. There are plenty of young volcanoes in the Cascades that haven’t blown their tops yet.
Who’s next?
Ecosystem Notes from Quinault Rainforest
Introduction
I’ve spent the past few months wandering the Olympic Peninsula with my attention fixed mostly on rocks—tilted beds, breccias, sea stacks, and the stories they tell about deep time. But along the way I’ve been noticing the living world with the same quiet fascination. I’m not a biologist and I don’t pretend to be; I can’t name any of the plants I pass. What I can see is how each organism plays a role in the larger system, the way geology shapes life and life responds in turn. These are simply notes from a wanderer paying closer attention.
I’ll try to remember to label these environmental posts as NOTES to avoid any confusion, especially on my part. This first post arises from a short walk on a semi-muddy trail through the Quinault Rainforest, on the Olympic Peninsula. I won’t have much to say about the photos, and all identification will come from CoPilot (aka ChatGPT). I’m certain its identifications will be better than mine after hours of searching the internet.
Quinault Rainforest in Olympic National Park
Plate 1. That raging stream about 50 feet below me is one of hundreds draining this temperate rainforest, which gets about 12 feet of rain per year. Note the ferns, which are everywhere, even in the temperate forests of northern Virginia. Ferns must be the most common plant in cooler forests. I am in a narrow strand of Olympic National Park that has never been logged. This is primordial nature, viewed by a geologist, but I’ll do my best.
Plate 2. Map of the Olympic Peninsula showing the areas I reported on in previous posts. I’m going to be focusing on Site A today, with a few photos from D.
Plate 3. I haven’t seen this anywhere else I’ve lived, not even northern Virginia, but they occur everywhere here in Washington. Apparently this is a common occurrence in rainforests, where the ground is a dangerous place for seeds. I couldn’t identify the species in this photo, but this practice is very common for hemlock.
Plate 4. This pile of debris is a large log turning into compost and supplying nutrients to a variety of plants. The top of the photo shows the base of a young tree growing out of all this chaos. According to CoPilot and the Olympic National Park map, this area has never been logged, so I am in wonder of this pile of “forest garbage”. Is that sandy soil I see? Where did it come from? I don’t know.
Plate 5. If you look close in the exact center of this photo, you’ll see daylight on the other side of the base of this unidentified tree. It’s about six-feet in diameter and covered with an epiphyte community of mosses and liverworts. Those aren’t leaves or fronds, but communities that mimic ferns–for their own reasons.
Plate 6. Here’s an example of a Western Red Cedar that has grown into a mature tree after being nursed by a stump. I guess it will eventually absorb the rotting stump and grow to full height, but this is the largest I’ve seen so far in the region.
Plate 6. This miniature ecosystem caught my eye, but I had to turn to CoPilot to get an idea of what’s going on. As a tree trunk decays it goes through five stages: 1) moss; 2) liverworts; 3) fungi; 4) shrubs; and 5) young trees. This one seems to be in stages 1-4. I didn’t see any seedlings on it. The shrub is probably huckleberry and the mushrooms a bracket fungus, probably Trametes or Stereum.
Plate 7. I am fascinated by these nurse trees after seeing species of fig trees in Australia that devour living trees, like a giant fungus or alien. These are nursing on dead trees, however, so it isn’t as gross; but this one is now standing on its own legs after the original stump has begun to collapse.
Plate 8. This is the largest Spruce tree in the world. It’s 191 feet tall and about 1000 years old. It is growing in a swampy wet land at the inflowing stream to a glacial lake, Lake Quinault.
Plate 9. I thought this was toxic waste until CoPilot took a look at the photo: this is a mass of frog eggs (probably northern red-legged frog). There were several more at the shallow, marshy wetland where a stream fed Lake Quinault. The water is so clear you can see the bottom, which is only a couple of feet down.
Kalaloch Beach in Olympic National Park
Plate 10. This one was a doozy. These objects are one-two inches long, thin, translucent, and oval in general shape. At first, CoPilot suggested insect wings (until I told it the size), then gull secondary feathers (until I said, “no way”), then settled on small fish cranial bones–e.g. the opercula, the bone that covers the gill. I asked for references, but it supplied me with titles and no links (how did it find them?). I spent longer on this photo than I wanted to. I don’t fault CoPilot for its ambiguous response because I found nothing when I looked very specifically. This phenomenon is either so common that no one bothers mentioning it, or infrequently observed that no serious beachcombers have stumbled across it. I’m going to have to agree (for now) with CoPilot that these are small bones from a school of fish that was decimated by either predation, coastal fishing, or disease and only these translucent cranial bones survived by floating, until waves concentrated them on this beach. This is the only beach where I saw them. I guess there are no easy answers to some questions–unless someone who reads this is a marine biologist.
Plate 11. I solved the mystery of the white objects on the beach (Plate 10). I spoke to an ecologist I know who suggested they are a Hydrozoa called Velella-velella, which floats on the ocean like a jellyfish. They are a colonial organism that is blown about by the wind. They don’t swim so they are easily blown onto a beach and carried by waves. (Here’s a good article about them.)
Cape Flattery
I reported on this amazing location in a previous post. You can scan that post to get an idea of where these photos were taken.
Plate 11. I don’t know if this living (it looked healthy to me) tree was stressed or not, but CoPilot thinks these are perennial bracket fungi, which favor environmentally stressed conifers. There were only a few on this tree. I noticed that this forest didn’t show nearly as many signs of decay as Quinault Rainforest, despite its exposed location.
Plate 12. I had to throw this photo in because the root growing out of the tree(s) on the left looks like a dog that got its head caught in a hole, and died there. Its limbs of limp. Overactive imagination, I know. Nevertheless, this is a bizarre image because the dog is lying on top of a mound of soil. I’d bet there was a stump there that has decayed because the trees visible in this image are both composed of multiple trunks. A large tree died here (like the dog) and these are its adopted offspring. I would add that Cape Flatters, which is part of the Makah Reservation, has never been commercially logged. This is old-growth forest and this is a naturally occurring phenomenon.
Summary
Moisture drives everything on the Olympic Peninsula, soaking old volcanic and sedimentary foundations until the forest grows straight out of its own decay. Fallen logs become elevated nurseries, their wood breaking down under fungi and mosses until they’re more sponge than tree. Hemlock, cedar, and huckleberry take root on these platforms, sending roots around stumps and into the thin soils draped over ancient bedrock. Even the beaches tell the same story: waves sorting bones, shells, and driftwood carved from headlands shaped by tectonics and storms. It’s an ecosystem built on slow collapse and constant renewal.
Acknowledgment
I am experimenting with using CoPilot (aka ChatGPT) to help as I pursue my growing interest in ecosystems. I have been up front about where it contributed. It has been a great help, as well as an inspiration; if not for CoPilot, I wouldn’t have had the time of inclination to add these ecosystem NOTES to Rocks and (no) Roads. In fact, I’m tired of this entire series of posts, for which I get no compensation other than sharing my observations of the world. As a final note, CoPilot wrote the Summary and I stand by it.
Now I have to think about more than just rocks…
Coastal Geology of Kalaloch Beach
This report is the geology supplement to my general post about the Olympic Peninsula. It focuses on the geology of the beaches because, to be honest, Quinault Rainforest was impenetrable, physically and geologically, without massive logistical planning and support. What I found was more than I expected.
Figure 1. Photo looking north along Beach 4 at Site B in Fig. 2. The tidal range is about 10 feet here and the primary sediment consists of sand with a substantial mud component. The rocks seen in this image are bedrock that protrudes above the veneer of beach sediment. The cliffs to the right comprise glacial till that is less than 2.4 my old (Ma hereafter) deposited by glaciers during the Quaternary geological period. This photo was taken near low tide. The dark color of the beach surface is caused by the relatively large clay component (i.e. mud).
Figure 2. Location map of today’s field area, which will focus on Site B.
Figure 3. Schematic of the Cascadia Subduction Zone as it appears today. The Coast Range, which includes the Olympic Mountains, consists of sediments and volcanics that have been scraped off the subducting ocean crust. The white arrows at the front of the image don’t reflect the actual geometry of these sedimentary rocks. We’ll get to that in a minute. Today’s post will focus on these sediments and discuss their original depositional environment during the Tertiary period (~65.5-2.4 Ma), as well as evidence for structural deformation.
Figure 4. This photo has been marked-up to highlight what might not be obvious to a non-geologist. First, older sedimentary rocks have been tilted to a high angle (dark blue bedding plane line) by faulting, as suggested in Fig. 3, and exposed to erosion at the surface. This erosional surface is shown by the yellow line. These Tertiary sediments were buried several miles beneath the ocean surface where the grains were cemented by heat and pressure. We don’t know exactly when this occurred or how long this process took, but they were later overlain by younger glacial sediments during the Pleistocene epoch (2.4 Ma to present). The age of sedimentary rocks can’t be pinned down unless they contain material datable by radiometric methods or, more qualitatively, by the fossils they contain. Sandy sediments don’t contain fossils very often and I didn’t find any radiometric ages in either Rock D or the USGS geologic map, so I guess the Tertiary date is the best we can do for now.
Figure 5. Close-up photo of the contact between the Tertiary rocks and the glacial till. Note that the cobbles aren’t falling out of the fine matrix sand and clay; this is because these young (>2.4 Ma) conglomerates have been partially cemented and, in fact, they were stuck to the older rocks below. This photo tells us a lot: 1) the older rocks were tilted at depth (maybe a mile or more) and then pushed upward as suggested in Fig. 3 until they were exposed to the atmosphere; 2) in a very short time they were covered by cobbles (~2 inches or less) that had already been rounded in a river, which takes several miles of transport; 3) the type of sediment varied rapidly, from coarse sand to cobbles, probably within decades if not years, due in part to periodic changes in surface drainage (e.g. the advance and retreat of glaciers); 4) this is a good example of the principle of Uniformitarianism–the present is the key to the past: we see similar processes occurring today along the many streams and rivers draining the Olympic Mountains. But what about those pesky Tertiary sandstones?
Figure 6. This photo of the Tertiary sedimentary rocks reveals packets of thin-bedded fine sandstone with intercalated dark layers of (presumably) mud. I didn’t see any cross-bedding, which would indicate deposition in a river or nearshore dominated by waves or currents. What is obvious that this ancient (~50-2 Ma) shoreline was receiving sediment from a distant source, probably the volcanic highlands of the modern Cascades (see Fig. 3). Let’s take a closer look at one of those wavy layers.
Figure 7. Close-up image of a thin layer (about 3 inches), showing what is called Flasier bedding. This is found in modern environments where high-energy (e.g. waves and tides) periods are interrupted by quiet times during which fine sediment can be deposited between ripples. It is indicative of a shallow marine environment.
Figure 8. These holes were evident throughout the exposure. They are problematic and probably originated in several ways; however, one plausible explanation is that soft-bodied animals like worms burrowed in the sediments and their burrows filled with sediment with a different composition (e.g. fecal pellets). These channels then preferentially eroded when subjected to the harsh nearshore environment in which they are found. The presence of such holes at the unconformity (see Fig. 5) proves that they predate deposition of the overlying glacial till. Unfortunately, such trace fossils don’t tell us anything about geologic age, only that this seashore was teeming with life.
Figure 9. I stopped at another accessible beach a couple of miles south, but still indicated as Site B in Fig. 2, where the rocks were sandstone but with very little clay. The beds seen in this photo were thick (1-3 feet). They were not deposited in a high-energy, environment like those we saw before; they were probably deposited on the shoreface below fair-weather wave base–possibly part of a nearshore bar or barrier island. After deposition and burial, they were tilted less steeply than those seen further north (Fig. 6). The unconformity is seen in the upper part of this image, but it doesn’t seem to have as many rounded cobbles as before (Fig. 5). I have no way of knowing the relative age of these two exposures; this Tertiary rock unit is approximately 3000 feet thick and represents millions of years. All we can say for certain is that this shoreline wasn’t that different from what we observe today. Uniformitarianism in action.
Figure 10. This exposed outcrop within the tidal zone reveals what are probably concretions of iron oxides, formed during early diagenesis. They superficially resemble fossils, but that is not the case. As the sediment was squeezed during burial, incompatible elements like iron formed irregular blobs in an otherwise uniform quartz sand matrix.
Summary
This coastline was part of the Cascadia subduction zone during most of the Tertiary period. Sediment, including a lot of sand, was eroded from rocks being uplifted further inland where volcanism was active, especially during the Eocene (55-35 Ma). Beaches like those we see toady were common, as well as the various depositional environments found along the Olympic peninsula, including fluvial and submarine fans, beaches, and cliffs. But no glacial sediment.
These sediments were buried as more material was removed from the rising orogenic belt, which included granitic rocks with lots of quartz. Eventually they were caught by the complex trench uplift (see Fig. 3) and scraped onto the edge of N. America. This involved faulting primarily, suggesting this was a fairly shallow process (folds occur at depth where rocks are ductile). The tilted rocks were eroded for millions of years; then, about 2.4 Ma glaciers (nothing to do with subduction) covered the region and deposited all kind of sediment: rivers, lakes, undifferentiated till, moraines, etc.
Just as the East Coast of America has been a passive margin for more than 200 my, the West Coast has been a convergent margin. Instead of being steadily worn away by wind and water, these rocks are rising out of the sea at about 2 inches per year. And what a ride it is…
The Olympic Peninsula: Quinault Rainforest to Cape Flattery
This is the first in what I hope will become a regular series of blogs from my travels around the Pacific Northwest (PNW) as well as trips throughout the world. This series accompanies my main blog, Rocks and (no) Roads, which reports on the geology of various places to which I travel. You could see I’m expanding my horizons; in addition to posting about general sights in this post, I will also be including environmental and ecological observations that are of interest to me. My background is in geology, but I’ve become aware of a lot more since moving to the PNW. So this is a learning experience, thanks in large part to the availability of expert assistance from CoPilot, Microsoft’s version of ChatGPT. I’ll be turning to it to identify plants and evidence of animal activity in future posts.
This is the base map I’ll be using in many of my posts from the PNW. The star indicates my base in Tacoma. Today’s post describes a two-day trip to circumnavigate the Olympic Peninsula. Although this area is the wettest in the contiguous United States, with annual rainfall up to twelve-feet, it was clear for the duration of my visit. The labeled areas indicate where the photos described below were taken.
Location A: Quinault Rainforest. Photographs can’t do justice to the experience of being surrounded by a temperate rainforest. There are many trails of different lengths, but I followed a shorter one (about one mile) that followed Willaby Creek for a while before turning into the depths of the forest. This image shows the creek rushing by about 50 feet below the trail as it races to the south shore of Lake Quinault.
Location A: Quinault Rainforest. This rotted log is about six-feet in diameter. It is representative of the ongoing decay and rebirth of the forest. I’ll talk more about that in another post. Note the ferns growing out of the decaying wood.
Location A: Quinault Rainforest. There are at least half-a-dozen species of giant evergreen trees in the rainforest, and I can’t identify any of them; but CoPilot suggests it might be a Coast Redwood, part of a local population found in this area. This example is typical in an area known for its “Champion Trees”. We’ll get to that in the next photo.
Location A. Quinault Rainforest. This is the world’s largest Spruce tree. It is 191 feet tall and about 1000 years old. It is one of the six champion trees located in Quinault Valley. The other champion trees are: the world’s largest Western Red Cedar, Douglas Fir, and Mountain Hemlock; and the largest Yellow Cedar and Western Hemlock in the U.S. This tree is growing in a wetland area at the NE end of Lake Quinault.
Halfway between Sites A and B. During my day-trips around the area, I’ve noticed a what appears to be a regional interest in post-industrial art, including but not limited to large concrete facilities that have been left to decay in place, as well as small sculptures and other curiosities scattered throughout the cities and forests. I don’t know if this is intentional or not: the roof of the information board collapsed (no doubt because of 12 feet of rain per year) and was set aside, possibly as a monument to nature?–or evidence of the poor funding and mismanagement of the National Park Service.
Location B: Kalaloch Lodge. This is the view from the deck of the lodge where I spent the night. Kalaloch Creek meanders as it approaches the coast and enters the Pacific Ocean here. An impenetrable pile of driftwood (actually logs and entire trees) has collected on a sand spit deposited on bedrock.
Location B: Beach 4 at Kalaloch. Steep bluffs of glacial till front a wide beach with many exposures of rock, which I’ll discuss in my next post.
Location C: Ruby Beach. This was the last beach before US 101 turned inland for ten miles. I took a video because a series of photos can’t possibly convey the beauty of this location as the sun was rising. Note all the rocks protruding from the water at low tide. The rocks are rising from the earth as we make our way north.
Location C. This selfie demonstrates how chilly it was, with the temperature below 40F. Rock formations like this are called stacks; they are erosional outliers as the coast recedes over millions of years, pounded day and night by rocks carried by waves; and tides that exceed nine feet–twice a day! You have to time your visit accordingly or all you’ll see is water.
Location D: Point Flattery. This is the extreme tip of the contiguous United States. The bedrock we saw further south, at Ruby Beach, now forms wave-cut cliffs that tower 40-80 feet above the waves. The top of the bluffs is 330 feet above sea level. The view from the catwalks is breathtaking. Vancouver Island, BC, is visible in the background, with peaks around 6000 feet.
More PNW Humor. All of the toilets we saw along the coast were constructed over septic tanks, with warnings about what to put in them. Someone with a sense of humor modified just one word of the official message…
If you get the chance, I strongly recommend visiting Olympic National Park, but you’re going to want to spend a week unless you live nearby in the PNW.
Gravel Beaches on Vashon Island
This photo of the gravel/mud beach at Fern Cove Nature Preserve reveals a marine delta, fed by Shinglemill Creek. Unlike the mud/sand delta we saw at Dash Point, the sediment here is dominated by rounded gravel and small boulders. Mud, sand and silt form the matrix. All of this material is available from the glacial till that comprises Vashon Island.
The inset map (right) shows the two sites I visited today. Fern Cove is located near the northern tip of Vashon Island whereas Maury Island Marine Park occupies the SE side of an island that is connected to Vashon by a fill zone about 200 yards across (which is eroding away as I type).
Fern Cove Nature Preserve
Vashon Island was logged out, so the beautiful evergreen forests that cover most of the island are all second growth. The state is trying to restore the original habitat, and Fern Cove is a good example; they don’t allow dogs!
I’ve noticed that people in the Pacific Northwest (PNW) appreciate industrial art of a practical nature. I found an example here, in a threatened habitat that is actively being restored. I guess this old truck body isn’t hurting anything; I’ve also noticed that communities in the PNW don’t waste money on unnecessary actions.
This photo shows how the gravel is localized within the delta, forming low nearshore gravel bars. The intervening areas are muddier than I found at Dash Point.
These gravel bars do more than concentrate larger rock fragments. Acorn barnacles encrust the larger ones, which are several inches in diameter. The tidal range is about 7 feet here and the beach is inundated twice daily, so I think these are living. I’ve seen barnacles on large boulders before, but I didn’t know they grew on stones. I guess the wave energy isn’t high enough to disturb them over a life cycle; their larvae must hang around after a storm and find a new rock to inhabit.
On all the beaches I’ve reported on for Rocks and (no) Roads, I’ve never seen this before. This is an active feeding area for sea birds. These are mussels, which the birds grab from a nearby mussel colony on the shallow delta front and drop on exposed gravel during low tide. The shells break open and the birds feast, leaving their dirty dishes behind.
This Western skunk cabbage is just emerging from the wetland surrounding Shinglemill Creek. It will grow leaves up to two feet long, but it makes a beautiful display in the unseasonably warm late winter we’re having in the PNW.
Maury Island Marine Park
On the opposite side of Vashon Island, I visited a gravel quarry that operated for about seventy five years, before being purchased by the state as a park. This is another example of letting nature recover without interference.
This photo looks SSE towards Dash Point. Commencement Bay and Tacoma are visible in the background; a little sunlight is shining on the bay. The cliff has been quarried for 75 years, so the 400-foot vertical face is now a steep incline with a trail that utilizes many switchbacks to reach the shoreline. Let’s see what I found…
There is no mud or sand visible on this beach. I thought that might be a side-effect of so much gravel quarrying, so I walked beyond the limit of the quarry; the substrate didn’t change, but there must be lingering effects after so many decades of preferential sand removal.
After what I saw at Fern Cove, I thought I was prepared for whatever I might find; but I never expected to see clam shells with the hinge lineament intact; this shell even closed after the bird (e.g. gull or crow) has eaten the soft tissue. The shell actually snapped shut after the animal was eaten. Amazing!
This spectacle gives driftwood a whole new meaning. Someone even built a crude shelter in the background from the logs that have collected over the years. There are a lot of trees in the PNW and many of them fall into Puget Sound, ending up all over the place. This looks like what coastal geologists might call a “hot spot”, a location where waves converge during storms.
After a hot climb, even in 45F temperatures, I made it back to the top of the cliff. This area was a barren wasteland in 1975, when the state took over; park authorities and volunteers have worked tirelessly to not only allow recovery, but to keep out invasive species. I think they’re doing a pretty good job.
Summary and Acknowledgments
I learned more biology on this day trip than in my entire life. For example, sea birds eat mussels on muddy beaches and clams on cobblestone ones. They carry the hard shells in the air and drop them on rocks, breaking them open to reveal the animal hiding within. Apparently I arrived at these two beaches just after lunch, when the tide hadn’t cleaned up the dishes yet.
I’m going to pay more attention to fauna and flora in my future posts.
I don’t know anything about biology, so all plants and animals identified in this post came from CoPilot (AKA ChatGPT). I didn’t check their identifications because it wasn’t worth the hours that would have required. This is not a research paper. I’m just going out into the world and observing through my dirty, discolored glasses.
But now I can see living things…
Active Delta Accretion at Dash Point State Park
INTRODUCTION
Puget Sound is a complex water body with many channels and bays, all of them within a couple of hours drive from my home in Tacoma. I haven’t posted much about this because I’ve been overwhelmed by so many active sedimentological processes that are neither marine nor fluvial. But I had to say something about what we discovered on this short trip, less than 30 minutes from our house. Because of the bluffs (~300 feet) surrounding Puget Sound, streams draining into the various inlets and bays are short and relatively steep. There are exceptions of course, such as the Nisqually River, which flows from Mt. Rainier to the southern end of Puget Sound; however, short streams are very common although many of them have been incorporated into municipal storm drainage systems. Today, we found one that was in an almost natural state.
Figure 1. This photo, looking landward from Dash Point beach, says it all: A small stream enters between the two bluffs, each about 200 feet in height, and dumps copious amounts of sand, silt and clay onto a mesotidal shoreline (the tidal range is about 7 feet), where the volume of sediment input overwhelms the nearshore wave and tidal regime to create an expanding delta.
Figure 2. (A) Regional map showing the complex, glacially sculpted, morphology of Puget Sound. The star is approximately where my house is. (B) Google Earth image from about 7000 feet showing Dash Point and the extensive delta being constructed by sediment delivered by a small stream.
Figure 3. The small stream feeding the delta has been confined to a stone-lined channel. This is the entire inflow. The water looks pretty clear, but appearances can be deceiving in the context of sediment transport. This is a bedload-dominated stream, which means that it is mostly transporting sand and silt, and minimal clay particles. The entire area surrounding Puget Sound is constructed of glacial till, which is mostly gravel and sand. A closer examination will confirm this inference.
Figure 4. This is the delta seen in Fig. 2B during an ebbing tide that is near its minimum. The expanse of the delta, at approximately the same tidal height, is seen here at ground level, revealing tidal channels and swales; but the surface is sand and silt with minimal clay restricted to the swales, like those seen in the middle of this photo.
Figure 5. The flattened sea grass serves as a current indicator that perfectly matches the ripple orientation. Note that the tops of the ripples are flattened by the high velocity of the ebbing tide. Waves weren’t very large on this particular day.
Figure 6. Sand bars are constantly being created and destroyed at the fringe of the delta, probably in seasonal cycles. I don’t know where a late February date fits into this cycle. Perhaps I’ll need to return in the summer.
Figure 7. The delta was less symmetric on this February day than the undated image in Fig. 2B. This image looks to the west. It reveals multiple shore-perpendicular sand bars, until the delta is interrupted by shoreline development. Nothing like this was present on the eastern side of the delta on this day. The sediment supplied by the stream in Fig. 3 is being transported westward in this shallow water, but not by waves like we saw on this day, which are perpendicular to the shore.
SUMMARY
Dash Point beach provided a great opportunity to study nearshore sedimentology. Just look at those ripples in Fig. 7! Despite being created and destroyed twice a day, we find rocks preserving similar ripples throughout geologic time.
With so much gravel in the Puget Sound area eroded from the glacial till (gravel and sand), it was exciting to see finer grained sedimentation occurring in real time. Gravel beaches are uncommon in the geologic record whereas rippled, sandy beaches are very common; after all, the previous source of this sand and silt was a glacial till, itself the product of erosion by ice and fluvial transport during the previous twenty-thousand years.
Material is recycled by our tectonically active planet.
Earth abides…
Quaternary Geology on Mt. Rainier
Figure 1. View of Mt Rainier from the west. At 14410 feet, it is the most prominent peak in the contiguous United States. It has 28 glaciers, with the largest total surface area in the lower states–35 square miles. Mt Rainier is a stratovolcano, composed of andesitic lava (rather than basalt), material ejected from the summit, and ash layers. This type of volcano is commonly found in subduction zones; they tend to have explosive eruptions (e.g. Mt Saint Helens). The oldest rocks on Mt Rainier are about 500,000 years old. It is active and listed as a decadal volcano–one of the most dangerous volcanoes in the world. Its last major eruption, accompanied by caldera collapse, was 5000 years ago, but minor activity was noted during the nineteenth century.
Figure 2. From my home in Tacoma (star) it’s a two-hour drive to Mt Rainier National Park. It is part of the Cascades Range, which comprises many well-known volcanoes like Mt Baker, Mt St. Helens, and Mt Hood.
Figure 3. This photo was taken on the south flank at an elevation of about 5400 feet, near the visitor’s center. It was a beautiful day and there were a lot of people preparing for some cross-country skiing on a couple of feet of snow. From this elevation it takes 2-3 days to reach the summit, almost 9000 feet higher. It’s hard to imagine it being so high and taking so long to reach.
Figure 4. These southern volcanic mountains are part of the Tatoosh Range, with peaks of about 6600 feet. Most of the volcanic rocks comprising these mountains are andesite, intermediate in composition between basalt and rhyolite. It is also very viscous, behaving like peanut butter and thus not flowing well. Andesite is commonly found at convergent plate boundaries where it is thought to result from mixing of basalt (from the oceanic crust), continental crust, and sediments accumulated in the accretionary prism.
Figure 5. Map of the 28 glaciers on Mt. Rainier. The glaciers fill canyons and valleys that were partly cut by ice. Figure 3 shows a smooth mountain, but in reality most of the smooth areas are the surfaces of glaciers. We’ll look at one below. The ellipse indicates the area discussed in this post.
Figure 6. (A) Narada Falls interrupts the descent of Paradise River, fed by Paradise Glacier (see Fig. 5 for location), making it drop a couple hundred feet over a thick layer of andesite. Andesite tends to form blocky flows, as shown in the right side of the photo, where the water seems to be climbing down steps. (B) Possible contact between younger volcanic and older intrusive rocks. Igneous activity within the area has been continuous for at least 50 my, during which time erosion has exposed older intrusive rocks like this granodiorite, which is part of a pluton intruded between 23 and 5 Ma. It is important to keep in mind that the volcanic rocks originated in plutons (magma chambers) emplaced miles beneath the surface. As they are exposed, new volcanoes form as more magma is injected into the shallow crust in a continuous process. (C) Differential weathering has accentuated layering in this volcanic rock, which was probably created by a series of ash layers deposited in quick succession–geologically speaking.
Figure 7. (A) View looking north towards the source of Nisqually Glacier (see Fig. 5 for location), which originates near the peak of Mt Rainier. The area delineated by the blue rectangle is the face of the glacier. (B) Closeup of the face of Nisqually Glacier. The characteristic U-shaped valley carved by glaciers is highlighted in white. Note the dark material within the glacier, probably wind-blown fine sediment. It looks like the face is a couple hundred feet high. I’ve never seen a retreating glacier before, so this is pretty spectacular to me. The Nisqually River originates right here…
Figure 8. View looking upstream along Nisqually River a mile downstream from Fig. 7. This is one of the most stunning photos I’ve ever taken because it reveals geological continuity, from the origin of a glacier 10000 feet higher, to the outwash being transported by a river. Amazing! Note the perfect U-shape where the shadow ends upriver. This area would have been covered by the glacier as recently as 10000 years ago.
Figure 9. Another mile downstream from Fig. 8 the walls of the valley have lowered, and are now rimmed by volcanic flows half-buried by detritus. Evidence of a glacier filling the valley has been erased by collapse of the valley walls. Rounded boulders fill the riverbed. The Nisqually River is overwhelmed by the huge sediment load and opens up new channels to continue flowing.
Figure 10. View looking upstream at the confluence of Nisqually River and Van Trump Creek. There are a couple of interesting features visible in this braided stream bed, less than two miles from the glaciers feeding each branch. The valley is very wide and flat-bottomed because it was carved by glaciers more than 10000 years ago. The large boulders (as large as three feet) covering the entire valley floor were transported by a glacier and became relict after its retreat because the stream flow, even during floods, is too weak to transport and erode them. The Nisqually River is cutting a channel through these relict sediments; the scarp is about eight feet in height. The white line delineates large, surface boulders from subjacent sand and silt with few boulders. Note that the surface boulders stop upstream where the white line curves sharply upward.
Figure 11. Image from 200 feet downstream of Fig. 10, showing a break eroded in the boulder-bar that crosses the stream bed at an angle. During recent heavy rain Nisqually River broke out of its current channel and created a myriad of flow structures such as the longitudinal bars seen in the lower-right of the photo. I think this bedform is actually a terminal moraine marking the maximum advance of a previous glacier–not necessarily the maximum glacial extent during the last two-million years.
Figure 12. (A) Andesite boulder (2 feet across) wet by recent rain shows fine-scale structure. The irregularity of the laminae, and phenocryst distribution, suggest to me that this sample represents ash fall rather than a flow. Magma with the viscosity of peanut butter tends to form smooth lines because it is difficult to penetrate, which would be necessary to create the mixed-up appearance between the lighter and darker shades in the center of the image. (B) Large block (~10 feet long) of intrusive rock similar to that seen at Narada Falls (Fig. 6B), but this is two-miles downstream. This relic was pushed/dragged by a glacier to this location. The white circle indicates where a close-up photo was taken. (C) Close-up (5x) image of the heavy block. It contains quartz (Q), plagioclase/albite feldspar (no orthoclase) (F), and amphibole (Am). My estimate of the composition is: 50% feldspar; 30% quartz; and 20% amphibole. Based on my estimated mineral composition, this would be granodiorite; however, I didn’t differentiate plagioclase and albite feldspar. (The former is darker than the latter.)
Figure 13. Map of potential volcanic risks associated with Mt Rainier–besides an explosion (e.g. Mt St Helens) and the eruption of ash which would cover a large area, depending on wind direction. Lahars (mud flows fed by all those glaciers) pose the greatest risk because andesite is too viscous to flow more than a few miles from its source.
Summary. I have seen evidence of continental glaciers in the Great Plains, the German Plain, and Ireland, but I never had the opportunity to observe glaciers up close. Alpine glaciers were nothing more than an abstract idea to me, something viewed from a distance.
I’ve looked out over the clouds from the summit of Haleakala crater on Maui, gazed into the cauldron of Kilauea, witnessed the boiling water rising from beneath Yellowstone’s seething caldera. I’ve seen videos of volcanic eruptions in Iceland, but I never imagined putting the glaciers and volcanoes together–right next door!
Usually, geology is observed as a series of images frozen in time, but at Mt Rainier it can be glimpsed as a real-time process that reshapes the earth’s surface–from top to bottom.
What a wild geological ride!
Deception Pass: Ophiolite or Volcaniclastic Sediments?
Now that I have a general idea of the geologic history of Northwest Washington (NWA) it’s time to start filling in the blanks. I immediately found a discrepancy in the rocks found on the northern end of Whidbey Island and the adjacent peninsula (Fig. 1): some authors identified these rocks as ophiolite–pieces of ocean crust that contain evidence of extrusion at mid-ocean ridges. Let’s see what I found.
Figure 1. Whidbey Island is a large island that is almost completely covered by glacial sediments. Its elevation varies from less than 100 feet above msl (mean sea level) to almost 400 feet, probably reflecting the presence of terminal morraines deposited as glaciers retreated. The box encompasses the study area, and the star is my home in Tacoma.
Figure 2. Progressively more detailed maps of Whidbey Island, showing the geological formations discussed in this post. (A) Deception Pass is a narrow channel (~200 yards wide) between Whidbey Island and a tiny island that connects it to a peninsula. Glaciers covered this entire area, advancing and retreating for at least two million years, while scraping out Puget Sound from unconsolidated sediments and easily eroded rocks. I entered Whidbey Island via a ferry from the mainland just north of Seattle. (B) Geologic map of the norther tip of Whidbey Island and the adjacent peninsula. The tan areas are covered by glacial sediments whereas the blue and green areas are reported as volcaniclastic rocks by Rock-D and the US Geological Survey national map. However, I read a Washington State geological report from 1962 that reported layered gabbro and other indices of a classic ophiolite sequence. Were Washington’s geologists incompetent? Over eager? Non-peer-reviewed? Or maybe they stumbled onto an exposure, described it in a state report, and, when no one cared, dropped it until they retired. I don’t know. Nevertheless, everyone is in agreement that the age of these rocks, which include basalt and marine sediments, is poorly constrained: they could be as old as 251 Ma, the beginning of the Mesozoic era; and as young as 66 Ma–the end of the Mesozoic. Wow! The thin, black lines indicate faults inferred from stratigraphic relations. The USGS has adopted the generally accepted paradigm of gathering legacy geological formations into larger groups that reflect the plate tectonic setting, rather than geographic location. The “official” description is so encompassing (including the kitchen sink) that it could easily include what the Washington geologic report called “ophiolite”. (C) Detailed map showing my approximate location along the path that followed the shoreline (blue circle) and the location where I saw these rocks in an excellent exposure (yellow diamond). I’m not sure why the rocks south of Deception Pass (blue) are differentiated from those on the north side (green) by Rock-D; the USGS map shows they are the same, as does my field examination (discussed below).
Figure 3. The ferry ride from Mulkiteo to Whidbey Island (see Fig. 2A) shows the same high elevation of Whidbey Island (left), Hat Island (center), and a promontory north of Tulalip (behind the ferry; see Fig. 1). This elevation of glacial sediments is approximately the same as the basement rock discussed below. This high relief shoreline is found throughout Puget Sound.
Figure 4. View looking north across Deception Pass, showing the nearly vertical bedding of rock layers, which could be original sedimentary fabric, metamorphic surfaces, or joints. Let’s have a closer look.
Figure 5. Images of the exposed rocks near the top of the hill overlooking Deception Pass (see Fig. 2C for location). (A) These medium bedded (~6 inches) layers contain clasts of different rocks. The height of the image is about six feet. (B) This twelve-foot exposure shows thin-bedded sand/silt layers with intercalated mud, but the layers are overturned (more than 90 degrees from horizontal) and reveal rapid changes in bedding, in addition to large boulders of material. (C) A bedding surface shows the characteristic sheen of schist, which adds metamorphic foliation to original bedding. These rocks are not heavily altered, so the layers seen in (B) probably reflect original beds. The clasts in (A) are matrix supported, which suggests mixing of different sized particles. This unusual texture occurs in environments where periodic mass failures (e.g. landslides or submarine slides) occur.
Figure 6. These images from about 100 feet elevation show more details of the rocks exposed at Deception Pass: (A) These layered sediments vary from medium to thick bedded and they also reveal gentle folding at this scale (the photo is ten feet high), suggesting that the overturned beds may be part of a large fold, possibly related to the faults seen in Figs. 2 B and C; note that the faults have a NW-SE orientation approximately aligned with the fold axis (I didn’t make careful measurements). It is also possible that the faults reflect brittle failure as the rocks were uplifted within the accretionary prism. (B) The left half of the exposure reveals a thick (> four feet) layer containing mixed clasts in an otherwise fine-grained matrix. The right side contains medium beds of a lighter material (We don’t want to get carried away here because of the vagaries of chemical weathering.) However, the contact between the two apparent lithologies is irregular; If I’m right about the left side being the bottom (questionable), this could have been a volcaniclastic slide (basalt, ash, pieces of rock, etc) that was then filled in by less catastrophic sedimentation. I admit I’m speculating, but we have to imagine life on the continental margin during active volcanism, during subduction.
Figure 7. This photo was taken looking west from Deception Pass (see Fig. 1), towards the Pacific Ocean. Those islands are emergent blocks of jumbled Mesozoic/Tertiary sedimentary and volcanic rocks. The last ten-thousand years of sea level rise has covered most of the evidence contained within the accretionary prism, leaving us with only fragments.
Conclusion. I didn’t see any ophiolite at Deception Pass, but there is plenty of evidence of accretionary tectonics; deeply buried sediments and volcanics were scraped off the subducting oceanic crust and deformed continuously. This process has to obey the laws of conservation of mass, which means that the accretionary prism accumulates further west with each passing year. What we see exposed at the northern end of Whidbey Island is occurring today beneath the continental shelf, all while the Juan de Fuca plate is swallowed by the earth’s mantle. There is a two-mile-deep trench about 100 miles off NWA, but it is filled with sediment eroded from N. America, unlike more-familiar deep-sea trenches (e.g. the Mariana Trench, about 7 miles deep).
All that sediment in the subduction trench is like sand in in your transmission: it gums up the works and slows things down, but the Earth cannot be stopped, its relentless, insatiable appetite for oceanic crust never satisfied. It will keep chewing up the Juan de Fuca plate until the upper mantle decides that enough is enough and stops pushing N America westward.
I can’t wait to see how this ends…
Coast to Coast
Plate 1. We’re not in Northern Virginia anymore!
I’ve relocated to the west coast–Tacoma, Washington to be exact, and that active stratovolcano looming in the background is Mt Rainier (aka Tahoma as it’s known to the indigenous people). The pristine water body is Commencement Bay at the southern end of Puget Sound. Mt Rainier is 14410 feet high, which makes it the most topographically prominent mountain in the lower 48 states; for scale, it is 43 miles from Tacoma, yet dominates the SE horizon. How did it come to be so close to the coast and yet so tall? Before I answer that question, let’s recap the geological story of NoVA that I pieced together over the last four years.
Over a billion years ago, NoVA was submerged beneath an ocean or marginal sea. Distant mountains eroded rapidly in a time before land plants. Vast quantities of sediment accumulated in layers of erosional debris that were subsequently buried by younger sediment. Between a billion and five-hundred million years ago, these sediments became rocks that were subsequently deformed as continental plates collided. They didn’t melt, however, and survived the cataclysm relatively unharmed, becoming schist and related metasedimentary rocks.
Plate 2. This schematic cross-section of the US East Coast is representative of NoVA. It shows the final closing of the Iapetus Ocean (forerunner of the Atlantic), which would have produced immense quantities of terrestrial sediments (e.g. the Devonian Catskill Delta in panel B). Unfortunately, these river and lacustrine sediments were subsequently eroded in NoVa as the ensuing mountains grew in size (panels C through E). They are today preserved in western New York State and eastern Pennsylvania. What survived in NoVA are older metasediments, ranging from ~1200 to 500 Ma, which were buried beneath the material eroded along the western margin of this figure (the yellow areas). Some Devonian intrusive rocks, intruded into older metasediments, survived along the Potomac River.
Plate 3. This reconstruction of the supercontinent, Pangea, coincides with Plate 2E. The square indicates NoVA. Pangea began to crack apart ~210 my ago, split by a spreading tectonic plate boundary, and for the last 200 million years, the older rocks have been slowly working their way to the surface as younger rocks were removed by erosion. They are now exposed to the elements and are weathering to form new layers of sediment in the Atlantic Ocean, beginning a new cycle.
What about the northwest coast, Tacoma and Seattle, you may ask?
Plate 4. This beautiful schematic cross-section represents the consensus opinion of geologists familiar with the Cascadia region. Tacoma lies at the northern end, near Mt Rainier. I will refer to this image frequently in my following posts. Study it a moment and you will realize that the geologic situation is similar to that presented in Plate 2A and B, but on the eastern (right) side of the closing ocean basin, i.e., the Pacific Ocean.
The Coast Range (including Olympic peninsula) is part of the accretionary wedge of a subduction zone, and Puget Sound lies within the forearc basin. In other words, I have moved from an extinct collisional tectonic regime to an active SUBDUCTION zone; Mt Rainier (Plate 1) is the tip of the geological iceberg, leaking sweat from the partially melting Juan de Fuca tectonic plate.
I hope you join me on this new geological adventure into the past…
Timothy R. Keen 
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