Geology of Carbon River Canyon, Pierce County, Washington
Today’s post reports on a brief survey of various geological features and processes that are revealed in a river that originates on the NW flank of Mt. Rainier. We’ll look at the types of rock found along its course, morphologic features, and discuss the processes that have created this natural area. We’ll also see some unintended consequences of deforestation.
Figure 1. Carbon River flows northerly before joining the Puyallup River and emptying into Commencement Bay, in Tacoma. This stretch is dominated by boulders up to a foot in diameter. However, the water is unusually muddy; we’ll find out why during our walk three-miles up the canyon that contains the stream.
Figure 2. (A) Regional map, showing Carbonado (pinned), a forty-five minute drive from North Tacoma. (B) Local map showing the Carbon River, which we followed for three miles. The river has cut a gorge that begins at Carbonado (star) and deepens to 250-300 feet at Fairfax bridge (triangle). (C) The geologic map from RockD reveals four major rock formations: the tan, which dominates around Carbonado is glacial drift, mostly sand and gravel; The pink outlining the river is Eocene (56-33.9 Ma) sedimentary rocks of terrestrial origin – rivers, lakes, etc; the yellow is Miocene-Oligocene (~23 Ma) fine-grained andesite. Additionally, the yellowish unit on both sides of the river is Pleistocene (2.5 Ma to 12 Ka) glacial drift. We didn’t see any of that today.
Figure 2. Our first stop was along the Carbon River near Orting (see Fig. 1A for location). River beds can be an excellent source of regional geology if they contain cobbles or boulders, which are good samples of source rock that may not be exposed.
This sample is of an intrusive igneous rock that contains lithic fragments (xenocrysts) of what looks like a darker, fine-grained rock. As magma moves upward through the crust, pieces of country rock are entrained, sometimes partially melting, but more often simple being captured by the cooling magma. This sample contains pieces of what I think is andesite. The host rock itself is probably also andesite, but its surface has been chemically weathered to form a white layer; this is a chemical process that involves the alteration of dark minerals like pyroxene and plagioclase into clays like kaolinite, which are light colored. Surface weathering also leeches out iron and magnesium, which darken a rock. This surface layer is usually only a few millimeters thick.
Figure 3. This sample tells a complex story. This is andesite (extrusive rock common to subduction zones), but any fractures within it were filled with magmatic fluids rich in Na, Ca and silica, by a later eruption. The result of this hydrothermal circulation is rocks that are neither extrusive nor intrusive; they are chemically altered by subsequent injections of magma at depths less than 1 mile, often even shallower.
Figure 4. Another fascinating example, which tells a different tale. This andesite block wasn’t injected with magmatic fluids beneath the surface; this sample is the result of gaseous magma erupting within a few hundred yards of a vent. The laminae tell us that the composition was changing subtly on short time scales; this was the top of the boiling magma, and thus there was a lot of trapped gas (e.g. CO2, H2S) that formed the vesicles after the lava had settled–but not yet solidified.
Figure 5. This irregular sample of andesite tells two stories: 1) The presence of phenocrysts (small white blebs at the bottom of the sample) implies slow cooling whereas the dark streaks imply a later stage of fast cooling, so that magmatic fluids with slightly different composition intermingled with the already hardening earlier stage. Imagine making taffy. 2) This sample wasn’t transported in the Carbon River more than a few miles. It would have broken along internal planes of weakness and become more like those we saw above. We aren’t far from the source.
Figure 6. This is an excellent exposure in a cliff cut by the Carbon River. It could be dismissed as undifferentiated glacial till, but I’d like to give it a second thought: What if this location records the movement of continental glaciers through Puget Sound (i.e. the Vashon till), in addition to alpine glaciers moving down from the Cascades? Could we even tell the difference?
The bottom of this section contains cobbles in a sandy matrix deposited horizontally. The middle part is finer, as seen in the absence of large cobbles, but a channel containing larger clasts is visible in the center of the photo, forming a “V” that points towards the bright spot in the modern channel, slightly right of the center of the photo. The depositional environment must have changed quickly to produce such a contrast in lithology.
Figure 7. This is a close-up of the center of the cliff in Fig. 6, focusing on the contact between finer, sandier sediment to the left, and coarse material in the upper right. The unconformity contact between them was created by erosion, either by a glacier or a river like the modern Carbon River. Note the change in bedding between the lower part of the exposure and the irregular layers above the unconformity. The lower section reveals a fining-upward sequence; but this was partially removed and the entire upper part of the section is dominated by conglomerate.
Figure 8. This photo was taken about halfway between Carbonado and the bridge (the star and triangle in Fig. 2B, respectively). The river was about 100 feet lower than the trail at this point. These bedded rocks are nearly vertical, as indicated by the dark, nearly vertical lines. They are Eocene terrestrial sediments that have been faulted and folded into their current orientation. They were probably deformed when the faults seen in Fig. 2C (thin lines on the geological map) were active.
Figure 9. Further along the trail we encountered this small landslide. A tree that had rooted in cracks within the sedimentary rocks fell along with a piece (about 10 feet long) of rock. This is a great example of how erosion is a continuous process that never rests, inexorably removing mountains and transporting them to the sea.
Figure 10. To the naked eye, this close-up of a recently broken piece of the slab in Fig. 9 looks like an intrusive, igneous rock, but it absolutely is not! Ignoring the dark coloration, which is either an organic stain or weathering product, I would say this is probably a coarse sandstone, possibly even a breccia or conglomerate. Breaking it with a rock hammer, to produce a fresh surface, would make identification easy; but we don’t do that in Rocks and (no) Roads. We use the geologic map instead.
Figure 11. This is the end of the trail, where the road crosses Carbon River across the condemned Fairfax Bridge — closed even to pedestrian traffic. The canyon is 250-300 feet deep here. We climbed a trail to the bridge before we saw the signs on the highway declaring it unsafe for men or beasts.
Figure 12. The canyon is very narrow at Fairfax Bridge, and Carbon River is forced into a bottleneck where it drops precipitously from the core of the Cascades. This entire canyon would have been filled with ice during the most recent glacial period. Compare this image, looking upstream, to those from the Nisqually glacier on Mount Rainier. That ice would have melted and advanced, pushing its way downstream, where it would have met the continental glacier covering Puget Sound.
Figure 13. We took the highway back to Carbondale. However, Route 165 is closed and the highway department isn’t maintaining it, so slides like this one are left for geologists to examine. This block is, according to RockD’s geologic map, sedimentary; it looks like sandstone and I didn’t examine it at the time. Note the reddish face on the large block facing the camera; red means this rock was deposited with a lot of oxygen, which means it was in a river and not the ocean, which has a much lower concentration of oxygen.
Figure 14. This photo presents a bizarre juxtaposition of geologic features. It was taken a few hundred yards downstream of Fairfax Bridge. The thick layer of sandstone to the left represents the unaltered strata, which has been tilted towards the NE, but not as much as the strata seen in Fig. 8. Such a sudden (spatially) change in orientation is probably why the geologic map (Fig. 2C) shows a fault along the river canyon.
Another interesting feature is the white patch in the center of the photo. I had several hypotheses about this but, after discussing it with CoPilot (aka ChatGPT), I’m going to go with the most plausible scenario: I think this is a layer of shale within the Carbonado Formation, which is both more easily weathered than sandstone, and a surface that can accommodate slippage on a fault more readily. The apparent bedding plane (i.e. the white surface) is an illusion, created by uneven weathering and the constant flow of water down an established channel.
Figure 15. I had to include this image. A block of sandstone, weighing more than a ton, rolled down the steep slope, hit the guardrail, bending it, before slamming into the asphalt road that was weakened by erosion, and smashing into the underlying, water-saturated ground. We noticed several spots where the culverts draining the road were insufficient, allowing water to collect and weaken the roadbed.
Figure 16. Another couple hundred yards further downstream (towards Carbonado in Fig. 2B), and we find thin layers of shale/sandstone nearly vertical, just like in Fig. 8. I didn’t measure my location precisely, but I wouldn’t be surprised if this is just above (uphill) the earlier observation. Note the layers of mud that resulted from differential weathering of clay layers within the Carbonado Formation.
Figure 17. I included this image to explain why the Carbon River was so muddy a few miles downstream (Fig. 1). The canyon wall to the west has been clear cut recently, allowing massive erosion of soil into the Carbon River. This is an ongoing practice in Washington with no attempts to rein in such destructive deforestation — all for the sake of profits. It just pisses me off!
Figure 18. The main point of interest in this photo is that the Eocene sedimentary rocks are oriented differently from those in Figs. 14 and 16; these rock strata are dipping WNW(I didn’t measure strike and dip), rather than vertical or ENE. Faults don’t occur at the surface, but only shallow enough for the rock layers to break rather than bend. Identifying and mapping crustal faults is as much an art as a science because we don’t have the remote-sensing tools to see what lies beneath the surface, whether it is glacial till or bedrock.
Summary
We saw a lot more on this trip than I’d imagined during the 45 minute drive to Carbonado. I’ll put it all together into a chronology and try to relate it to other nearby locations we’ve visited. Let’s start as far back as the rocks will take us.
During the Eocene epoch, about 50 million-years ago, this area was covered with streams, lakes, and, presumably, forests. There were no rivers like the Carbon back then — the sediment consisted primarily of sand and silt, with a smattering of clay. These sediments were eroded from an active volcanic complex. Further west, along the Pacific coast of the Olympic Peninsula, contemporary rocks include turbidites deposited in deep water, probably submarine fans, and tectonic breccias. This tectonic environment has persisted to the present day. We examined one of these volcanoes, from the Miocene (~23-5 Ma), at Pinnacle Peak. We didn’t see any evidence of a nearby volcano today; instead, we saw several boulders that revealed what was going on within the magma chambers beneath the Cascades volcanoes.
Subduction didn’t end during the ensuing millions of years, but the earth changed. Ice sheets appeared during the Pleistocene (~2.4 Ma), covering the PNW while alpine glaciers formed in the valleys dissecting the volcanoes. Finally, about 500 thousand years ago, a really big magma chamber constructed Mt. Rainier; and Carbon River began to wind its way from the flanks of this monstrous volcano to the Puget Sound trough during interglacial periods. This entire canyon would have been filled with ice during cold periods. It was during this period that Carbon River would have emerged from the eroding, ice-covered terrain. It didn’t cut its canyon but inherited it, and transported the remnants of a permanently altered terrain.
During glacial retreat, thick blankets of sediment were deposited by rapidly flowing rivers fed by the glaciers. This occurred repeatedly for two million years, culminating in the most recent glacial deposits seen in Fig. 6. However, this process of glacial advance and retreat occurred within both the vast continental glacier and the alpine glaciers, resulting in overlapping layers of poorly sorted sediment.
One final word: the muddy water we saw in the Carbon River is probably caused by the clear-cutting of the steep slopes defining its canyon; however, glacial melt during the summer transports a lot of fine material from beneath the glaciers covering Mt. Rainier. When we visited the Nisqually glacier and viewed the headwaters of Nisqually River in December, the water was crystal clear. Maybe the water turns opaque every summer.
It has been a wild ride through the last 50 million years along the modern Carbon River.
The Flora of Point Defiance Park
This beautiful, mature second‑growth coastal temperate rainforest is only ten minutes from my house. I should get there more often, and perhaps I will after taking an interest in ecosystems in addition to geology. Today’s outing was an opportunity to begin acquainting myself with the ecosystem I inhabit, so I made an effort to identify several plants along Five Mile Drive, a road that circumnavigates the forest and is permanently closed to auto traffic. When I started walking along this path, this forest was nothing but a lot of green trees and stuff. My perspective changed over an hour or so, and about 2.5 miles.
This colorful maple tree is a native to Puget Sound. It earns its name because those deeply lobed leaves are up to a foot across. But what are its roles in this forest? CoPilot had a lot to say about that. The Big Leaf Maple is fundamental to this ecosystem because it creates a huge amount of leaf litter that contributes to the forest floor and thus supports a vast microbiome, as well as a nurturing environment for conifer seedlings. This large tree (50-150 feet) fills in gaps in the forest canopy, supplying shade and catching rainwater, as well as dew. Its dappled shade creates a diverse understory for new growth. It is important in the success of birds, insects, bats, squirrels, and deer. They all hang around it and depend on its large mass for homes and meals. Its large root system contributes to hydrologic stability by stabilizing slopes, slowing runoff (and erosion), retaining moisture, improving soil infiltration.
This large tree is native to SE Europe and they were introduced after the logging boom led to immigration. However, it is not an aggressive invader; this lovely tree does not form thickets, outcompete conifers, spread wildly, alter soil chemistry, suppress understory diversity, or create monocultures. In CoPilot’s words, “It’s a polite non-native.”
This nondescript shrub gives the forest its lush, evergreen understory identity. It is one of the top berry producers in the Puget Sound lowland forest but, more importantly, its berries ripen later in the season, extending the berry window into early fall. It also thrives in the acidic soils Douglas fir and hemlock create, in turn moderating soil conditions. It bridges the gap between ecological disturbance (i.e. logging) and late-successional forest. One could say that it serves as a node in the forest’s ecological web. This is an unrecognized keystone species in the understory.
Oregon grape is the huckleberry’s silent partner in keeping the understory secure because it is also an evergreen shrub. In addition, it supplements the food web by blooming in March-April, providing nectar when few other shrubs are in bloom. Its berries ripen by mid-summer, bridging the gap between early berry producers like salad and late huckleberries. It precedes huckleberry in forest succession but persists as the canopy creates a shadier understory.
Himalayan blackberry is less like the fruit-bearing huckleberry and Oregon grape, and more like the Trojan Horse. The fruit are abundant and sweet, produced for a long season, and loved by all the denizens of the forest; thus, they are spread like wildfire, colonizing the entire forest–without predators in Pt. Defiance. The result of this invasion includes impenetrable thickets, root masses that choke other species, spreading by long, arching canes, and monocultures. It kills all the other stabilizing shrubs and creates a single mass of Himalayan blackberry. If left unchecked it can destroy an entire ecosystem, including the conifers. Holy shit!
This nitrogen-fixing tree is the first responder when the forest is disturbed. As such it enriches soil, accelerates forest recovery, feeds the next generation of conifers, increases understory diversity, boosts fungal activity, and generally improves growth rates of proximal plants. Alder is the first tree to reoccupy damaged land, preparing the site for the conifers that will come later. Wherever alder stands exist, the biodiversity of the forest is increased. Because it gives so much to the forest, alders only live 40-60 years, thus acting as a continuous healing agent as a forest recovers from an environmental shock. This is the pioneer tree of the lowland forest. I hope it can defeat the Himalayan blackberry.
This view of the Tacoma Narrows bridge through the thick forest reveals a green landscape perched atop high bluffs overlooking Puget Sound. As far as the eye can see, from this vantage point, the forests were clear-cut to make profits and fuel the economic development of the Pacific Northwest.
But at what cost?
Mima Mounds Natural Area Preserve
It was unbelievably gorgeous weather here in the Pacific Northwest (PNW), with a high of 74 F, so we took a look at a couple of nearby natural areas.
Mima Mounds
This exceptional area isn’t unique but it is an unsolved geological problem. It is also peculiar in being a transition zone between active prairie and forest ecosystems. I’ll try to explain this with photos, and some help from CoPilot, Microsoft’s version of ChatGPT. However, I won’t solve the geological conundrum, which may never be fully explained.
The Mima Mounds are about 16500 years old. They formed as the last continental ice sheet retreated from Washington. The mounds you see in this photo sit atop a gravel surface created at the southern extremity of this glacier. People, including the native Americans who have lived here for tens of thousands of years, have been perplexed by this bizarre topography.
There is a 1.9 mile trail that goes through the area; however, when European Americans first came here, this topography extended for more than twenty miles. Most of it has been demolished.
This is a portion of the information board at the interpretive center. (A) The Mima Mounds were constructed after the ice sheets began to retreat for the last time. The inset map shows the location of the NAP relative to the glacier and modern cities. This was the absolute furthest south of thick ice, but that doesn’t mean it was warm. (B) This is a rare photo of a cross-section through a mound. The dark soil is organic-rich and excellent potting soil. The subjacent glacial outwash is gravel. (C) This is a photo from the air, which shows their regular spacing. This is the geological problem. How could they have formed over about 400 square miles?
There are many theories for the origin of the Mima Mounds, and other mounds found in N. America and elsewhere. These are summarized on the Wikipedia page. I included this photo because it was on the poster; and the size/shape of these blocks of permafrost from Scandinavia are a good match to the mounds we saw. And they are in a similar, post-glacial environment.
Prairie Ecosystem
I’m discussing both the geology and ecology of Mima Mounds together in this post because they are inextricably connected. There are two ecosystems competing for space in this lumpy prairie environment. The result is there for anyone to see, if they ask the right questions.
This low shrub was identified by CoPilot as snowberry, which is a staple of the prairie. I only saw them on the mounds, their roots in good soil, but apparently they also do well in the sandy glacial soil. From my perusal of the internet, I can’t disagree.
My ignorance is hilarious. I thought this was some kind of wildflower … but CoPilot identified it as a Douglas fir seedling. They were growing only on the mounds, sometimes in clusters. This is evidence (to me and CoPilot) that this is a dynamic transitional environment where forest species coexist with prairie flora.
This fern caught my eye because it is not a natural inhabitant of a prairie ecosystem. However, the mounds are near a forest, so … CoPilot thinks it is a bracken fern, which is native to the area but not a prairie environment. It often behaves as an aggressive colonizer in disturbed edge habitats. That last phrase gets my attention because this preserve is at the edge of a mature forest.
This lovely flowering shrub was identified by CoPilot as a species of Lotus, commonly called Spanish Clover, deervetch, or trefoil. It grew in the sandy areas between the mounds. My quick check can neither confirm nor deny this identification, but this common variety of clover is native to the PNW.
This mound is populated by bracken ferns, crowding onto the area with good soil.
I don’t know what species this copse of young trees is, but it is obviously encroaching on the prairie ecosystem.
You never know what you’re going to find when you go outdoors. According to CoPilot, this is probably the result of NAP’s policy of not removing human artifacts that don’t interfere with the environment. Manpower shortages, policy priorities, etc. Mima Mounds was established in 1976, so it’s fair to say that this structure is at least that old. I guess NAP hasn’t gotten around to it yet, which fits my nascent sense of priorities in the PNW.
Bill Frank Jr. Nisqually National Wildlife Refuge
We stopped for a quick visit, which turned into a one-mile death march (for me, after walking 4 miles on uneven ground), to see where the Nisqually River empties into Puget Sound. It originates from the face of an alpine glacier above 6000 feet on Mt. Rainier. This is a tributary stream that forms a series of algae-covered ponds.
Here we are, less than a mile from the Nisqually River’s delta in Puget Sound. There is a lot of downcutting and the creation of sand bars, but no boulders or even cobbles. This glacial river has been tamed by nature over a distance of about eighty miles, but it is still flowing pretty fast. We didn’t make it to the delta … maybe next time.
Summary
This was a great day trip to a fascinating and beautiful area, where the prairie and the forest compete for space on the top of mounds of top soil whose origin is a mystery. This back-and-forth movement of plants on timescales of years to decades suggests that the PNW is alive and well, the flora responding to minute changes in soil and air temperature/moisture, precipitation, wind, etc. Let’s hope these two thriving ecosystems can continue their dance without further human interference.
I have my own hypothesis about the formation of the Mima Mounds, which is consistent with the facts. 1) As the glacier melted and retreated, over thousands of years, shallow lakes developed in local depressions. These lakes were shallow, perhaps twenty feet deep, and limited in extent. 2) Fine-grained sediment (i.e. mud) and organic debris settled in them to depths of no more than ten feet. They were ice covered for part of the year. 3) As the glacier retreated further, streams began to superimpose their beds onto this landscape, but they weren’t like a glacial river (e.g. Nisqually River). These were weak streams flowing over a post-glacial landscape, meandering and not cutting new channels. 4) This mild erosion was superimposed on a landscape dominated (in this area) by an antecedent pattern like that seen in the Arctic (see above photo). Fine-grained sediment was removed, following a suture pattern until only irregular bumps remained. 5) This process of sheet-flow erosion continued to the present day, leaving us with these paradoxical mounds.
I am a sedimentologist, so I think in terms of turbulence and flow as interconnected processes that alter a landscape slowly, one grain of sand at a time. There wasn’t enough turbulence to strip away the veneer of clay over this basin in the time allowed, so it chipped away at the weak edges of lumps of soil that were probably held together by the roots of plants.
In other words, the thawing of permafrost created the pattern, and turbulent flow polished it to what we see today.
Prove me wrong…
The Geology of Franklin Falls
Washington has more than 3000 catalogued waterfalls, so we stumble onto them regularly. Waterfalls form wherever there is a change in the lithology of the crustal rocks: along crustal faults, which naturally create vertical planes; where weaker rocks lie beneath strong ones and are reached by a downcutting river; and of course where glaciers have sculpted the land. These natural features create spectacular views as well as revealing glimpses of deep time.
Plate 1. Franklin Falls is about 70 feet high. The reddish rocks forming the cliff are Eocene sandstones and volcanic rocks (56 – 34 Ma). The park follows the South Fork of the Snoqualmie River along I-90. The parking lot is quite large and we found a parking space, even though it was great weather; more than one million people visited the site in June.
The rocks in this photo don’t reveal well-defined bedding although there is a suggestion of bedding to the right side of the image. However, I think the bedding marker is the vertical, dark streak that is slightly wavy. This curvature suggests that these rocks were deformed while ductile (buried deep enough to bend rather than break). The geologic map identifies an anticline in the area, and I think I found it.
Plate 2. Franklin Falls is located just west of Snoqualmie Pass, at an elevation of about 3000 feet. The Northern Cascades are very rugged and access is limited, especially in the winter. The topography shown in this map reveals steep slopes, especially through this pass, which I assume is the easiest route through this rugged terrain.
Plate 3. The geologic map from Rock D shows three distinct rock units, which reveal the general geological history of this area. The oldest rocks are part of the Jurassic melange belt (201 Ma), comprising metasedimentary and metavolcanic rocks that were originally deposited in deep water, possibly on a submarine fan. These sediments were buried and then scraped off the subducting ocean plate. This process deformed them as if they were put in a blender; hence the name, melange.
Almost 150 million years are missing before Eocene volcanic and sedimentary rocks were deposited in a continental setting, possibly a river or lake not too close to mountains; the rocks seen in Plate 1 don’t contain any large boulders like we see in the modern river bed below them.
Granodiorite is an intrusive igneous rock containing light-colored feldspars like albite, and slightly less quartz than granite. This batholith was emplaced during the Miocene (23-5.3 Ma) into rocks much like the Eocene rocks exposed along the river (Plate 1). However, granodiorite magma does not produce andesite, which is the most common volcanic rock in the Cascades. All those plutons being shoved into the upper crust exhibit a lot of chemical variety because the magma mixes with continental crust as it rises.
The final piece of the puzzle is the normal fault shown on the geologic map. The NW block of melange is labeled with a “U” to indicate that it moved upwards relative to the SE block of sandstone/volcanics. This displacement brought the older (Jurassic) rocks upward, eroding the rocks that were deposited above them. The fault isn’t shown as extending into the granodiorite, however; it is very difficult to identify a fault (usually by displaced stratigraphic units) in relatively uniform intrusive rocks, which contain no layering. If the field geologist saw evidence of this fault extending into the granodiorite, they would have used a dash line to indicate uncertainty. This line stops cold…
Plate 4. This photo is looking NW, across the fault, towards the melange. This peak is a block of Jurassic rock that was once tumbled in the accretionary wedge after it was scraped off the subducting ocean crust. The peak further back is Denny Mountain, Oligocene (34-23 Ma) volcaniclastic deposits. They are older than the granodiorite. We couldn’t hike around these mountains to see the rocks up close, but fortunately, gravity has made our job easier.
Plate 5. These are small boulders of the Jurassic Melange that rolled down the steep slope (i.e. face of the fault) and landed in the river bed (see Plate 1). (A) Mud deposited in the deep-sea trench was buried deeply enough to squeeze quartz and feldspar into veins (thin white lines in the image). However, it is probable that the thick layer of light-colored minerals were injected along weak points when the Miocene granodiorite was emplaced. Note that the vein becomes more orange to the right of the sample; this is probably a local concentration of K-feldspar (e.g. sanidine). (B) This sample looks like it was spun in a blender because the thin layers of light-colored minerals are twisted rather than approximately following original bedding (which has been obliterated).
Plate 6. This rounded boulder of granodiorite is about 18 inches in diameter. The large, white phenocrysts are plagioclase feldspar with a low Ca content whereas the darker ones with blurred sides are hornblende. The matrix is fine-grained plagioclase with a higher Ca content (feldspar composition varies with potassium-sodium-calcium). Calcium is associated with dark, potassium with pink, and sodium with white-colored crystals.
Summary
Today’s field trip was a unique opportunity to integrate the large scale observations (Plates 1 and 4) with hand samples (Plates 5 and 6) using a geologic map (Plate 3) as a guide to understanding the geologic history of the Cascades Range. Let’s try to create a simple, plausible geological history from what we know.
I’m going to list a series of geologic events because I don’t feel like finding/creating schematic cross-sections to pictorially demonstrate what I’m saying. I’ll start from oldest and proceed to the youngest.
- About 200 Ma: Muddy sediments were deposited in a subduction trench, which was approximately aligned N-S along the axis of the later Cascades range. Over the next 10-20 Ma these sediments were buried miles beneath younger erosional debris, until they were snagged by the overlying NA tectonic plate and deformed like putty.
- Between 120 and 60 Ma: It is unclear exactly what occurred, but subduction ceased, probably because several large crustal blocks slid to the NW along a series of transform faults. The melange we see in Plates 4 and 5 missed the boat and remained buried, slowly inching their way to the surface.
- About 60 Ma: Subduction resumed and a series of volcanoes produced lava, which mixed with sandy sediments in a continental environment. Volcanism continued for 30 Ma, creating a mixture of fresh lava and sandy sediments, which were buried while erosion continued — a delicate balance of tectonic uplift and isostatic sinking.
- Between ~30 and 23 Ma: Tectonic uplift increased, bringing the Jurassic melange and Eocene rocks (Plate 3) into the crust’s brittle fracture zone, driven by a combination of subducting plate dynamics and upwelling magma, as the ocean plate melted. This agglomeration of different rock types began to fracture along contacts, while still deforming plastically internally. The normal fault seen in Plate 3 would have occurred along such a seam during this complex exhumation process.
- A series of hot, rising plumes of magma originating at the top of the subducting ocean slab eventually reached these rocks, possibly within a few miles of the surface, between 23 and 5 Ma. The granodiorite filled every fracture and fault, creating the complex pattern seen in Plate 3. The thick veins of quartz/feldspar seen in Plate 5A would have been injected during this interval.
- Exhumation has continued, from 5 Ma to the present, modified by glacial scouring of the ancestral Snoqualmie River canyon. Today we see these rocks conveniently frozen in time, from our perspective.
Identifying the contacts between these many rock facies is a laborious task that will take decades, if not centuries, to complete. Nevertheless, it is obvious that a lot has occurred in the last few million years. The earth’s surface is a conveyor belt on which the pile of soil/rock is constantly removed by wind/rain/snow/ice…
That’s my story…
Geological Survey of the Columbia River Gorge
The popular route east from Portland, Oregon, is I84 following the Columbia River, which cuts across the Cascades range. There are plenty of scenic views and geology to examine, but few safe places to stop. Thus we followed the Washington shoreline along state route 14.
The inset map shows the distribution of volcanic rocks within Washington and Oregon. The oldest are predominantly andesites erupted from volcanoes (triangles) within the Cascades between about fifty and five million years ago (Ma), shown in light brown. The bright green represents the Columbia River Basalt Group, which flowed from fissures between seventeen and five Ma. The youngest rocks are primarily andesite erupted from volcanoes within the last million years (e.g. Rainier, St. Helens, Hood, Baker). The rectangle shows the area we are traversing, which contains a mixture of these rocks.
We stopped frequently, but I’ve lumped the photographs into four areas: 1) Beacon Rock is near the beginning of Columbia River Gorge; 2) Lake Bonneville and 3) Hood River give a good picture of the central canyon; and 4) Columbia Hills is where the river enters the gorge before cutting through the thickest section of volcanic rocks.
1. Beacon Rock
This photograph looks east towards Beacon Rock, which has an interesting origin. It was originally injected into a cinder cone volcano about 60 thousand years ago (Ka). Subsequent, multiple glacial floods eroded the loose material away, leaving the core, which is called a neck. This region was never covered by continental glaciers, although there is evidence of alpine glaciers like those still existing on the high volcanoes (e.g. Rainier or Hood). During numerous advances and retreats of continental glaciers into Canada, large lakes formed and periodically drained catastrophically. These floods, which were as deep as 1000 feet, naturally followed the Columbia River to the Pacific Ocean.
This low road cut reveals a thick layer of volcanic rock (basalt, according to Wikipedia) overlain by volcaniclastic rocks, which are loosely cemented. That’s why the DOT placed netting over the friable layer. These are sedimentary rocks consisting of volcanic ejecta as well as material transported by water.
According to Wikipedia, Beacon Rock is 848 feet tall and there is a trail to the top that is popular with hikers. It doesn’t look that high from the bottom, but I’m glad I didn’t trust my first impression and climb it; as stubborn as I am, I would have made it–and wished I hadn’t for the next week. It looks a little pale to be basalt, including the boulder visible at the bottom of the image; in a terrain with continuous volcanism, spanning the gamut from rhyolite to basalt, for 50 Ma, you just can’t tell from surface features. Some basalt is a little lighter colored and some andesite is darker–it’s a spectrum based on mineralogy, not color.
This eroded slope got my attention because it reveals an interesting juxtaposition of an exposed basalt outcrop that is rounded (unlike the earlier exposures we saw) and light-colored boulders of much smaller size (less than three feet). These rocks are too uniformly light in color to be weathering of basalt or andesite. There is some rhyolite (a leucocratic extrusive rock found within the Cascades) in the region, but an alternative explanation is that these are flood deposits from the aforementioned glacial lakes. There are many deposits from these mega floods within the gorge, but I couldn’t (easily) find a map of them. Anyway, this is what I would expect to find in such a sedimentary deposit–mixed rock types that are rounded by transport tens, if not hundreds, of miles during flooding episode. The bedrock would be rounded by collisions with these boulders. If the shoe fits…
2. Lake Bonneville
The central part of Columbia River Gorge is characterized by several broad valleys with sediments filling the margins of the canyon. This is a typical exposure from this area. The rock looks like basalt to me; the map (see first plate) shows a mingling of volcanic rocks along the river, which would have been a low point for lava to flow towards. However, this is not a volcaniclastic deposit as we saw before; instead, there are several, heavily weathered (i.e. smooth) flows of lava (3-10 feet thick). The lowest layer seems to be dipping towards the camera as if flowing down a steep slope. Maybe…
3. Hood River
This location is close to the eastern entrance to Columbia River Gorge, where flood basalts erupted from multiple fissures in the crust. In other words, there are no nearby volcanoes and steep slopes; thus, the basalt flowed over a relatively flat landscape, forming rolling hills. This photo reveals basalt flows that gently slope to the left, as seen in the middle-right and background of the image. These massive flows partially blocked the river many times–long before glaciers dominated the landscape. The island in the center of the channel is a remnant of one. I haven’t heard of any glacial lakes in this area, however, so the blockage must have been partial–these thick sequences of basalt didn’t occur at one time, but over millions of years, giving the ancient Columbia River time to erode passages through them.
4. Columbia Hills
Columbia Hills is the eastern end of the gorge, where the Columbia River ends its meandering path to the Pacific. The rocks are basalts erupted from many fissures between 17 and 5 Ma. According to the latest interpretation, these rocks were ejected from the same mantle plume that now underlies the Yellowstone caldera in NW Wyoming. They have nothing to do with subduction or the Cascades volcanic belt, even though the much younger Mt. Hood (in the background) towers over them.
We are now in Eastern Washington, a climatic zone with completely different characteristics than west of the Cascades. This volcanic range creates a rain shadow and resulting precipitation is less than 20 inches here; and it shows in the scrubland ecosystem. These extensive basalt flows are no longer covered by younger andesites from the high Cascades (the young volcanoes like Mt. Hood).
The volcanic layers are thin and extensive (see the map at the beginning of this post). They include columnar joints as I described in a previous post. The textures seen in this photo reveal the variability of lava coming from a single source; for example, individual, blocky layers cap this exposure whereas the rock presents a ropy texture lower down (middle-right of the photo).
Summary
The Pacific Northwest (PNW) didn’t exist before the Tertiary period, which began at 65.5 Ma. However, Pangea began to split apart at about 200 Ma, which should have created plate collisions here because the N American plate would have necessarily overrun the plates comprising the ancient Pacific Ocean. The west coast of N America was located approximately at the WA-ID boundary. So why don’t we see Jurassic and Cretaceous volcanoes and their associated volcanic deposits in the PNW?
This question has perplexed geologists for decades. After carefully collecting data from far and wide, a still-controversial theory has evolved: For more than 100 million years, this tectonic collision was accommodated by transform faults (e.g. the San Andreas fault system in California). A tectonic plate collision is not a conveyer belt as shown in schematic representations.
This schematic profile of the PNW shows several transform faults, which misalign the Pacific mid-ocean ridge (note the misalignment of the dark, Juan De Fuca Ridge. This tectonic scenario didn’t develop until those transform faults, which were not perpendicular to the mid-ocean ridge, could no longer accommodate the displacement of these microplates with N America. That apparently happened about sixty-million years ago. Some of these slivers of volcanic terrain have probably become exotic terranes that are now part of Alaska.
That is probably why we didn’t encounter any Mesozoic ((251-65.5 Ma) volcanic rocks within the Columbia River Gorge. Instead, we discovered a Tertiary volcanic landscape dominated by andesite/basalt lava flows, preserved because the transform faults had stopped absorbing the collisional, crustal tectonics. A real subduction zone emerged from this chaos and created the Cacades.
Superimposed on this was the unexpected (tectonically speaking) effusion of basalts as the westward-propagating N American plate rode over a mantle plume, which buried the evidence for this slipping history beneath miles of volcanic rocks. I can’t say anything else about this because I’m not actively researching the PNW’s geologic history.
My last word is that I can’t wait to see what new discoveries the PNW holds for me.
A Day Trip to Yakima
We wanted to see what was on the other side of the mountain, so we headed east from Tacoma and crested the Cascades at Chinook Pass. It was snowing at the pass and fog/clouds obscured what were probably majestic views. We pushed on to Yakima and found the Cowiche Canyon recreation and conservancy area.
This is a typical view from the trail, which follows an old rail line that was used for seventy years to haul apples out of the area. I have discussed the columnar basalts and vegetation in previous posts. It was late spring and all of the plants were showing their color while water rushed past in the creek at the bottom of the canyon.
This undulating columnar basalt caught my eye because of the color and its wavy appearance.
The Burlington Northern railroad wasn’t afraid to use explosives to create a path through what was described at the time as, “A dry, rocky canyon good for nothing except a railroad.” The rails have been removed, but the eleven bridges required to construct a rail line to cover the 3 miles of the trail system are mostly still in place.
Here’s one that didn’t last. It was replaced by a pedestrian bridge constructed by the Cowiche Canyon Conservancy and the Bureau of Land Management.
We took a more circuitous route back to Tacoma using US12 through White Pass. It didn’t snow, and we got a look of one of the many water falls in Washington.
This was the best angle I could get, and I still couldn’t see the bottom! It turns out that Washington has more than 3000 catalogued water falls, more than any other U.S. state. Clear Creek is rather small at only 300 feet, so I guess there is some more air down there.
It was a beautiful day in the Pacific Northwest, including the snow flurries. A ten-hour day trip took us from coastal Washington, over a 6000 foot pass, into dry eastern Washington, where we hiked through a canyon filled with native plants and rocks (hahaha), and back over the Cascades past a magnificent water fall in Wenatchee National Forest.
This is undoubtedly the most beautiful place I have ever lived…
Ecological Notes from Cowiche Canyon
Cowiche Canyon Recreation Area is located on US12 just west of Yakima, Washington. The region receives 9-14 inches of rain per year, making it a dry area; thus, the trail system includes both shrub steppe (uplands) and riparian (along Cowiche Creek) habitats. We followed the main trail along the path of a rail line that was in use between 1913 and 1984 along the creek; however, the wetland is very narrow, in places constricted to less than 100 yards. Thus, I encountered plants from both environments.
The canyon walls are composed of a series of basalt ledges with intervening slopes covered by talus and colluvium, which are part of the shrub-steppe habitat. I discussed the geology of the area in another post.
The recreation area is maintained by the Cowiche Canyon Conservancy in partnership with Bureau of Land Management. This stone is a piece of the columnar basalt that lines the canyon.
It’s fortunate that I visited this area during spring, which lasts a little longer here in the Pacific Northwest. As always, I used CoPilot (AKA ChatGPT) for identification while I try to remember scraps of the huge amount of information presented in this mixed environment.
This is Asclepias speciosa, also known as showy milkweed. It is native to Yakima county and is a host species for Monarch butterflies.
The leafy shrub with dark leaves is snowberry–Symphoricarpos albus (or possibly S. oreophilus, which also occurs around Yakima).
The low, brightly colored shrub with straight stalks is probably wax currant (Ribes cereum). The bright green is small leaves and the small patches of pink–barely visible in the photo–are the flowers. These are both native plants.
CoPilot wasn’t so sure about this, but it might be Creek or Red-osier Dogwood (Cornus sericea). This specimen was growing in the bottom of the canyon, not far from Cowiche Creek, which is a natural location for this native riparian species. It will probably become a small tree.
This is my favorite from the walk. Silky lupine (Lupinus sericeus) is one of the signature wildflowers of eastern Washington. I sure am glad we caught them in bloom.
This looks like Pale‑stem buckwheat (Eriogonum heracleoides), another native wildflower to the shrub-steppe habitat.
My untrained eye thought this was Pale-stem buckwheat, but CoPilot pointed out the different leaf pattern and color. This is (probably) Sulphur Buckwheat (Eriogonum umbellatum), another common wildflower to Yakima County’s uplands.
Antelope bitterbrush (Purshia tridentata) is a foundation species of the steppe. This young one had lots of flowers, but the old ones have bare branches; and groups of them grow and die together in cohorts after a disturbance like a wildfire. Yet another native plant.
After some discussion, and sharing a close-up, CoPilot swears (hahaha) this is Woods’ rose (Rosa woodsii). However, its justification fits what I see with my own, somewhat confused eyes.
Here’s a close-up of the fruit. The shrub is covered with small nuts that have a distinctive shape, and are definitive for a wild rose. This is another native species to the steppe habitat of Eastern Washington.
This photo, looking across Cowiche Creek, puts it all together for me. On the other side of the canyon we see columnar jointed basalt, several plant species similar to snowberry, bitterbrush, and buckwheat. Along the creek are dogwood and wild rose; and in the foreground is (maybe) big sagebrush (Artemisia tridentata).
When I took this picture, all I saw was a bunch of plants. After carefully examining them with CoPilot, it has become a mixed riparian-shrub-steppe habitat. However, I didn’t see/hear any birds or other animals, even though it was a cool day with temperatures in the mid-sixties.
CoPilot was a great help, but it is not infallible–more like working with someone who has studied some biology/ecology. After all, it is only a Large Language Model, not an AI system trained on recognizing plant species. Nevertheless, it was a great collaborator and I learned a lot from our collaboration.
Volcanic Rocks at Cowiche Canyon
Introduction
Burlington-Northern Railroad built a line through Cowiche Canyon in 1913 to transport apples, but it was abandoned in 1984 and the land was acquired by the Cowiche Canyon Conservancy for a non-motorized vehicle trail system. The main trail extends 2.9 miles along the South Fork Cowiche Creek, crossing the 11 bridges constructed for the railroad line.
The left panel shows the distribution of Columbia River flood basalts, deposited between 16 and 6 Ma. Yakima and Cowiche Canyon are outlined by a rectangle. These volcanic rocks were erupted in overlapping flows with erosion and landslides occurring between individual layers, which are irregular and not shown in this map. The ages from the USGS national geologic map are Tertiary (66-2.6 Ma). Tacoma is marked by a smiley face.
The right panel shows the Cowiche Canyon trail system and the specific area discussed in this post. The stream itself hosts a riparian habitat whereas the uplands comprise a shrub-steppe environment.
Observations
The canyon walls consist of a series of ledges like this with eroded slopes between them. The ledges are erosional margins of basalt flows and the slopes consist of talus and fine sediments weathered from the mafic rocks.
As we traveled west up the canyon, columnar-jointed basalt began to appear in the ledges overlooking the trail. Several pieces had rolled down the slope into the stream bed; these were about three-feet in diameter. Columnar jointing results from slow cooling of a uniform basalt flow, which causes joints to form hexagonal blocks like these because of thermo-mechanical failure during a decrease in volume.
These semi-circular blocks got my attention because, if you look closely, they appear to be eroded hexagons I estimate to be more than six feet in diameter. These are very large basalt columns that have either toppled or…
The top of this photo shows a birds-eye view of columnar basalt blocks because of their horizontal position. The size varies from smallest on the left to the largest blocks on the right. I reported on similar, horizontal columnar joints in a previous post and proposed that the lava flowed down a slope before solidifying.
The lower-right part reveals columnar basalt in a vertical position. This juxtaposition suggests (if my model is correct) that the lava from multiple flows covered an irregular landscape–sometimes flowing into canyons like Cowiche Canyon, and somtimes over fairly level ground.
This remarkable set of columnar joints got my attention because of their undulating form. I’ve never seen anything like this before. This style of jointing (supposedly) results from uneven cooling and weathering; for example, a heavy load on the layer during cooling leads to pinching and swelling at fairly uniform spacing. That sounds reasonable to me.
Another weathering feature of these rocks is the fissile structure revealed in this image. More solid blocks are interspersed with flaky layers, possibly (I’m speculating here) associated with necked and wider segments of an individual column. For example, the wider sections might undergo shear during stretching, resulting in microscopic shear layers within the minerals comprising the original lava. These weak layers would permit water to penetrate and weather the mafic minerals of which basalt is made.
These highly weathered columns are more than six-feet in height. They suggests an alternative mechanism, shear from flowing as the basalt cooled; this might disrupt the microscopic structure without interrupting the macroscopic jointing process. Maybe…
This photo really got my attention. It reveals horizontal columnar joints abutting vertical ones in the upper-center of the image. There’s a lot going on as hot lava flows over an irregular landscape, but I think this is a vent where more magma flowed out; not a large eruption, but enough to have a separate cooling history from the rock it penetrated. I should note that the Columbia River basalts flowed from fissures rather than point sources like volcanoes. The entire area shown in the map above was cut by fissures that led to a shallow magma chamber, which is still down there although it has probably solidified by now. Or not…
I like this picture because it reveals how much weathering can change the appearance of what was once molten lava in only a few million years. Note the layer of angular blocks sandwiched between weathered columns.
Conclusions
This is a typical basalt column that isn’t as weathered as some of the others. All those shards I’m standing on resulted from the breakdown of the rock by water seeping into its internal structure, where it altered the mafic minerals (e.g. pyroxene, plagioclase feldspar, biotite), which are susceptible to chemical weathering. This is where all the mud in the world comes from.
It was a great day to drive over the Cascades at Chinook Pass, where it snowed on us (in June), and explore the Columbia plateau. I’ve never seen so much variability in basalts before. The magma chamber underlying central Washington was a giant chemical reactor that released pressure by erupting a mix of fluids that cooled to form minerals and then these magnificent rocks. These rocks tell us how the magma chamber evolved over several million years; and once they were exposed to the atmosphere, they began to record the slow process of being reduced back to their basic constituents (fine-grained minerals like clay), which can remain suspended in water and begin their long and perilous journey to their final resting place–sometimes a lake but, ultimately, the ocean.
Everything eventually returns to the sea…
Point Defiance Rose Garden
Spring is a great time to be outdoors in the Pacific Northwest, so we visited the rose garden at Point Defiance park, which is more than 130 years old. It had a full-time gardener until the 70s, who lived in a house on the grounds. I forgot to take a photo of it. Not very exciting, but I found myself drawn to the amazing colors and shapes of so many flowers in bloom at the same time.
This is the central pagoda of the rose garden, which is arranged in concentric rows around it. The design philosophy of the first master gardener (Ebenezer Roberts) was more natural rather than highly organized, and that’s how the garden has remained through time.
This jumble of color is typical of the rose garden on a spring day.
They had more than roses in the rose garden. This is Jerusalem Sage or Phlomis fruticosa,. It appears to be in full bloom.
I never suspected there was a state society for a flower, but they had planted rows of numbered plants in a portion of the rose garden. The fence is to keep out the deer, which devour, stomp, and basically destroy flowering plants.
The rose beds are arranged concentrically around the center, but they are mixed up by variety. I’ll just show some photos of the gorgeous blooms we saw.
This Dublin Bay variety is distinctive because it has strong stalks and grows high, concentrating growth on the ends of very thorny branches.
I didn’t take a photo of the sign identifying this remarkable rose, but CoPilot thinks it is Scentimental (1997, floribunda).
They even had agave doing very well in the PNW. We want some of this for our yard.
This is a Korean dogwood (according to CoPilot). Note the deer-protection installed around the base. Deer are very destructive to new growth and weak trunks, based on my personal experience.
And those aren’t flower petals, they are bracts. The tiny flowers are in the center.
Note the dark purple, almost black, flowers growing from the same plant as the orange flowers.
Amazing!
These are the plants that don’t require deer protection. They are outside the defensive fence, and they are thriving.
I thought this was a Japanese garden until…
I noticed dolls arranged throughout the area. It reminded me of the horrifying Disney World ride, It’s a Small World. Now that song has returned to haunt me. Creepy … if you want my opinion.
I wasn’t very excited about going to the rose garden at first, but the combination of perfect weather and all these treasures of nature on display won me over. This is yet another reason to love living in the Pacific Northwest–especially Tacoma.
A Visit to the Pacific Bonsai Museum
Small, carefully pruned trees are fascinating. I went to the National Arboretum when I lived in the Washington DC area, so I had to see what was on display here in Tacoma. The Pacific Bonsai Museum is mostly open air, with a few sensitive examples in a greenhouse. They leave the trees out year-round and have skilled bonsai arborists trim them according to their creators’ expectations.
I learned that there are different schools of thought on bonsai, from growing trees and shrubs from seeds, to working with dying plants, even stumps, and painstakingly reviving them. Then there’s cultural differences that vary between nations. However, bonsai began in China where it’s called “penjing”. It means tray scenery. The museum had their displays arranged by country of origin. I didn’t take notes and my photography is spotty. Still, here are some beautiful examples of bonsai.
The Bonsai Museum is in Federal Way, Washington, and is supported by Weyerhaeuser, the lumber company; it’s located on a large campus they built and then sold to someone. It is free and there is no apparent security, except from the Rhododendron Garden next door. I guess vandalism hasn’t been a problem; who would want to destroy such beautiful antiques?
This is a wisteria. If you’ve ever dealt with this out-of-control vine (I have), you will be amazed that this old example (note the size of the trunk) is so tidy yet produces the distinctive flowers. The specimens on display ranged in age from the late nineteenth century to the seventies.
This is part of the Japan exhibit. I think these are all trees, probably juniper–a favorite among bonsai artists.
As an example of what good hands these fragile trees are in, this specimen was created by a past director of the museum. It was my favorite. I like the idea of rejuvenation from near death.
European Olive, in training since 1969, from Italy.
You can see the difference between the trees/shrubs grown from seeds or youth (and those that are saved from the dust heap) in this recovered specimen. I love the intermingling of dead (white) wood and living tissue.
This German artist created a forest, but the individual “trees” were falling over so there are wires holding it together. I imagine there are a lot of failures in practicing bonsai.
This successful “forest” is by an Australian. Note the “rocks” at the base; they are actually dead knobs from a large stump. Also note the very shallow pan: this is truly penjing–tray scenery–and not a potted plant.
The Rhododendron Garden
We crossed the gravel plaza and paid a visit to the Rhododendron Garden. The casual stroll through every imaginable color flower was worth the price of admission, even on a chilly spring day in the Pacific Northwest.
I’ve commented about “nurse logs” before. Here they are part of the landscaping.
I love the flowers peaking out of one end of this log while a bush is struggling to escape from the other end.
Rhododendron is one of the most common groups of plants, native to every continent although most of them originate from SE Asia. They do well in the Himalayas–not so much here in the PNW.
This caught my eye as we left the museum/garden…
What the hell is growing out of the top? I’d love to see what kind of flowers this tall shrub will produce later this spring…
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