The confluence of Sugarland Run (left) and the Potomac River (right). This is where today’s trip ends. The trees haven’t recovered their foliage yet because it is late March. We’re going to start upriver about a mile and end up here.

The last post covered the area inside the blue ellipse. We encountered 200 Ma shales and sandstones of the Balls Bluff shale. we expect to see similar rocks today, but we’ll be entering the Potomac flood plain. The red ellipse is where we are in this post. The numbers will be referred to later as the locations where photos were taken. The dashed lines for “rock” and “gravel” are approximate locations where the bed of Sugarland Run changed composition. It should be interesting.

Here at Site 1, the bed consists of large, angular boulders with rounded corners. These rocks didn’t travel far, probably eroded from now-gone cliffs like we saw upstream. This location, as with similar rocky transits we saw upstream, represents a point where the stream flows over an exposed ledge of bedrock. This is very common for streams in this area.

View looking downstream (north) at Site 1. Note the dramatic change in stream bed composition. The bar on the right consists of silt and gravel. Note also the eroded, soft bank on the left. We have entered the ancestral Potomac flood plain.

At Site 2 (see map above for location) gravel bars like this were found where a smaller stream entered Sugarland Run. It is probable that the current stream is cutting through ancient sediments because there is no source for gravel like this anywhere around. These are recycled deposits.

Confluence of Sugarland Run and a side channel at Site 2. Channels like this criss-cross the ancient flood plain. These are larger than those we saw further upstream on the Potomac in a previous post.

This photo from Site 2 is the last appearance of bedrock in the stream bed. Note the flat surface across the stream that tilts slightly towards the camera. This is a bedding surface for the Balls Bluff Siltstone. Upstream, these rocks form low cliffs and are tilted away from the modern stream. It is likely that overlying beds have been eroded after tens-of-millions of years by the ancestral Potomac River. The change in dip suggests, further, that there is a structural feature between this location and a mile upstream. There is some evidence for a fault that runs along Sugarland Run several miles upstream. It was probably part of regional adjustment during uplift over the last 200 my.

As with other streams on the Potomac River flood plain, there has been rapid erosion. This example from Site 3 can be dated by the age of the tree. I don’t know how old it is, but it is certainly less than a century. What is unusual is that this erosion is occurring inside a bend. Usually streams cut on the inside of a meander and deposit point bars on the outside. We’ve seen this at every scale in previous posts. From what I’ve read there has been rapid erosion in the last few decades because of urbanization. We saw an extreme example in the last post. The field data suggests that Sugarland Run is widening but not meandering. This is not a natural process in unconsolidated sediments like these. The ancestral Sugarland Run certainly does meander (see map above), but this rapid erosion unaccompanied by channel migration is not natural.

There are several small lakes near the modern Potomac River, such as this one (just north of the Site 3 label in the map above). Sugarland Run passes it within 100 yards, through unconsolidated muddy sediments. Features like this are difficult to understand because the age relationship between the stream and lake cannot be unambiguously identified through radiometric dating. Both developed in sediments of the same age, older than either feature. These lakes (see map above) don’t look like oxbow lakes. Given the common occurrence of depressions throughout the area, which form small ponds and lakes during the wet season, the geological fact that the Potomac floodplain has wandered across a wide swath of the area (see for example a previous post), and the lack of any outflow to a modern stream (see map), it is probable that these lakes represent undulations in the ancient flood plain and Sugarland Run is younger. It just happened to miss the lake as it cut down through the soft sediments without meandering.

This meander at Site 3 shows how Sugarland Run is becoming incised rather than following a typical meandering trajectory, as at Horseshoe Bend on the Colorado River. The scale is drastically smaller but the processes are similar; the stream lacks the energy to erode the banks and becomes “trapped”, so it cuts downward as the upriver source is uplifted relative to the outflow. In addition, this small stream appears to be widening, as seen in the eroded tree on the bank in a previous photo.

Another interesting feature we saw between Sites 3 and 4 was a couple of elevated flat surfaces like this one, seen in the center-left of the photo, about halfway between the current stream bed and surface. These benches were small in area (less than 100 feet) and at the current water level of the stream. My best guess (a common occurrence in geology) is that they were point bars when Sugarland Run was smaller and are relict features on the modern Potomac flood plain.

Here we are about 100 yards from the Potomac. There is no delta associated with Sugarland Run but there is a bar at its mouth (see first photo).

Sugarland Run is an intermediate-sized stream flowing into the Potomac River. Goose Creek is one of the larger ones, which supplies drinking water for the region, whereas Horsepen Run is a small one. Despite the difference in flow between these tributaries, they display similar geomorphic features (e.g. meandering, point bars, gravel and muddy beds, recent erosion and entrenchment) because they all cross the wide, ancient Potomac floodplain composed of mixed sediment types. The modern Potomac River itself is less than four-million years old although there is evidence of the ancestral river flowing though this area back 20 my. The supply of sediment has decreased over the eons as the ancestral Appalachian Mountains eroded, so we don’t see the kind of sedimentation today that would have been occurring several million years ago.

The sediments being eroded by modern streams like Sugarland Run record a climate and topography very different from what we see today. However, the physical processes were the same and the landscape was shaped, ultimately, by geological processes occurring deep within the earth’s crust. These same constraints produced the ice age that is closing in our times and associated fluctuations in sea level, adding nuances and new themes to the unfolding story of our Earth.

This adventure followed Sugarland Run (aka Creek) a mile or so, where it flows through a wide spot between ledges of sandstone. The area has been developed for a long time and the creek is crossed by weirs (white water in this photo). I don’t know if they were to maintain water levels or as roads.

This geologic map from Rock-D shows the starting point our walk (blue circle), which ends at Route 1582 (Algonkian Pkwy). The pink rock seen to the left of the creek (tan color running N-S) is Balls Bluff siltstone. It consists of of predominantly shale but we’ll see some coarser sediments today. These sediments were originally deposited in lakes during the early stages of rifting of the supercontinent Pangaea about 220 Ma (million years ago), when the modern Atlantic Ocean was first opening. We encountered this rock at its type locale, and again in Goose Creek.

This is our first glance of Balls Bluff siltstone. Note how it holds up the ridge that borders Sugarland Run.

Exposure of cross-bedded siltstone about twenty feet above the creek level. Note that it is dipping to the west at less than 30 degrees west. The strike follows the regional trend of ~30 degrees northeast. This is the orientation (rotated over the last 200 my) of the rift that tore Pangaea apart.

This close-up of the previous image reveals a well-sorted sandy texture. There are no large pebbles or angular rock fragments visible and it was rough to the touch. Like sandpaper.

This is a nice view looking up-section. The total thickness represented in this side-creek/drainage channel is more than 50 feet, which is not available in exposures elsewhere in the area. The lower part (note the stream bed near camera) is blocky sandstone whereas shale predominates up the section (noted by slopes rather than ledges). The sandstone/siltstone beds become thinner up-section but are present.

We didn’t only encounter 200 Ma lake sediments along Sugarland run; this photo shows a fire hydrant and a road that has been eroded by recent erosion. Apparently, someone wanted to keep the hydrant because the bank has been stabilized with blocks of riprap. Note the rounded, angular boulders lining the creek bed. They were eroded from the nearby cliffs.

The stream bed is defined by periodic rapids (see above photo) and pools of quiet water, as seen in this image. The flood plain is a couple hundred yards across here and the creek is meandering in soft sediments. The underlying rock is not uniform, which leads to this alternating pattern.

This photo shows the thin-bedded coarser sediments (upper left) at the top of the section.

The thick-bedded sediment in the lower section can be seen in this image to consist of both cross-bedded layers (lower right) and massive beds (just above the center and left-lower). This variability could be due to thin layers of mud, which weathers easily distributed irregularly when the sediments were deposited; or the thick layers could be channel deposits, for example. This kind of variation at the outcrop-scale suggests a dynamic environment; a likely scenario is rapidly changing channels at a delta where a stream originally entered a lake. These channels can change during a single flow event (e.g. a heavy rain) or every few years.

Many of the processes recorded in these rocks are at work today. This image was taken from a gravel point bar, looking upstream. The gravel clasts were rounded and probably were transported many miles from upstream, in the recent past. They are not from the original sediment, but were broken loose from exposed rocks within the last few million years and transported to this location. Note the large, fallen tree forming a temporary dam in the upper part of the image.

Because of the obstruction seen in the previous photo, flow and sediment delivery downstream is reduced temporarily (until the tree rots and collapses). The creek bed is exposed, showing multiple channels that predate the obstruction. Creeks never follow a single channel like a canal. Sugarland Run is a braided creek at this locality. This morphology is dynamic because of high sediment load and can change rapidly, unlike a more stable anastomosing river. The Potomac is an example of the latter. Of course, a river can change character in different sections of its channel, which we’ll see in my next post.

Much of Sugarland Run’s banks are deeply eroded, reflecting rapid erosion because of regional uplift. The previous image shows this on the east (right) bank. However, erosion and deposition in braided streams occurs at many time and space scales. This photo shows erosion of the stream bed on very short time and spatial scales, probably in response to the reduced sediment flow caused by the fallen tree obstructing sediment more than water.

At the end of our walk we met Algonkian Parkway, where the Balls Bluff sandy sediments have been removed (or were absent to begin with), creating a wide flood plain near a point where another creek joins Sugarland run (not shown). This major boulevard follows a natural rise to the south, the dividing point between highlands and the ancient flood plain of the Potomac River.

Today’s walk followed a braided stream about a mile between ridges supported by sandy sediments that were deposited about 200 Ma in lakes, when the supercontinent Pangaea began to split apart. It is very likely that this stream is following an ancient fault zone associated with that event. The orientation of these rocks and their lack of folding supports the inference that this area was being stretched and the rocks, which were still buried many miles beneath the surface, fractured to accommodate the crustal extension. In fact, intrusive rocks cut through the Balls Bluff sediments elsewhere in the area.

The sedimentary processes occurring along Sugarland Run today are not that different from when these sediments were first deposited in lakes more than 200 my ago. There is one critical difference in their depositional regimes, however; this region is experiencing uplift today whereas this was a sinking basin when Pangaea was torn asunder. Consequently, the original sediments were fine-grained, eroding from distant mountains whereas the gravel and boulder seen today is the crumbling remains of those ancient sediments.

We’ll see what happens when Sugarland Run reaches the Potomac next time …

I wrote a post about streams traversing the Potomac River flood plain in a previous post. Horsepen Run is a meandering stream that has cut down several feet across the gently undulating sediments blanketing the mile-wide flood plain at this point on the Potomac (Fig. 1).

Figure 1. Map of Horsepen Run area. The reference to Fig. 2 is from the original post.

It had rained for 24 hours prior to this field trip, and then temperatures plummeted to well below freezing. It never snowed but there was some sleet. We are going to see some interesting features that resulted from this unique event. The watershed for Horsepen Run is rocky, with bed rock never more than a few feet beneath the surface, except on the floodplain (Fig. 1).

Figure 2. Shallow ditch along the path leading to the Potomac River, just entering the black-circled area in Fig. 1.

When I first came across the curious ice structures in Fig. 2, I thought someone had ridden a bicycle along the ice for fun. I didn’t figure it out until later.

Figure 3. Meander in Horsepen Run. The surface appears to be ice. Note the ice lying along the opposite shoreline. A curious structure.

Figure 4. The surface of Horsepen Run where it empties into the Potomac is solid ice. We tossed a branch out and it broke, indicating the ice is several inches thick.

Figure 5. Close-up of Horsepen Run at the Potomac. Note the broken ice about one inch thick lying along the opposite bank.

It’s time to put it all together. The water level during the steady rain was elevated so close to the stream’s outlet. The temperature was low enough (~10 F) to freeze the surface while water continued to flow beneath the ice. Even though the temperature remained very low, when the floodwater ran out from under the ice, it cracked and collapsed like a pane of glass. The newly exposed subsurface water at the lowered level then froze. This process occurred even in a few inches of water (Fig. 2).

The ice sheets (Figs. 4 and 5) serve as a high-water marker that melted away with the next thaw, serving as markers of how much water can collect during a light rain on nonporous soils and rock.

Geology integrates rock and soil with the atmosphere and hydrosphere into a holistic system that can surprise us at every change in the weather.

We had a chance this week to see what the Nature Conservancy does with our donations. They buy land and either maintain it or return it to the state or local government as public parks.

FIGURE 1. This post takes us back to the Potomac River, where we hiked around Fraser Preserve, a plot of land owned by the Nature Conservancy and open to the public. The photo above shows a stream flowing under an old concrete bridge as it cuts its way to the Potomac River. Downcutting here was similar to what we’ve seen elsewhere along the VA side.

FIGURE 2. The blue dot marks the gravel road that leads to the trail we took, which is indicated by the dashed line. The light-green area to the left is the schist (1000-511 Ma) we’ve seen along this stretch of the Potomac. The darker area to its right is a metagraywacke from the same era. These rocks were originally deposited in an ocean trench where ocean crust was being subducted beneath continental crust. They were buried along with the ocean crust and deformed into medium-grade metamorphic rocks.

We left the nature preserve and briefly entered Seneca Regional Park (north of the road marked DCWA in Fig. 2) to get access to the Potomac River.

FIGURE 3. A side channel of the Potomac River with a gravel bed and pristine water flowing over outcrops of schist. The elongate dark areas are lenses of schist on the river bed. Similar features were observed at River Bend park and discussed in a previous post. The banks here are gravel and would be a great place to cool off on a hot summer day.

FIGURE 4. This is a view of an abandoned channel of the Potomac River, taken from the top of a steep bank, probably 40 feet above the river. This area is about a half mile downstream of Fig. 3. The water seen through the foliage is part of a cut-off lake that is active only during high-water. Similar features have been seen further upstream but with some water flow year round, as discussed previously.

FIGURE 5. Close-up of a small exposure of metagraywacke along the access road at the blue dot in Fig. 2. Note the thin bedding and striations aligned perpendicular to the hillside. These are probably sole marks that indicate the flow direction in the original sediments. These rocks appear as lenses within the larger volume of schist, which was originally deep-sea mud. Imagine submarine flows flowing down the steep face of a submarine fan as turbidites.

This is a short post because we have seen most of these rocks and geomorphic features before. The novel feature that prompted me to write this was the wide floodplain (at least 300 yards across) and totally abandoned channel (Fig. 4). I also haven’t seen such clear water with no mud deposited at the shoreline. This location isn’t far from the narrow chasm that created Great Falls, where the river turns southward. I also noted a large number of steep gullies that appeared with no warning, indicating recent erosion from the surrounding hills, which are a couple hundred feet above river level. It seemed that some of the higher ridges were supported by cobblestones rather than bedrock, a feature we noted in Claude Moore Park that suggests ancient point bars.

Photographs can’t capture the complex topography of Fraser Preserve, especially with such colorful foliage interfering, so I encourage anyone who has the time to get out and see this beautiful landscape for themselves.

One final note: I support Nature Conservancy in their efforts to preserve natural lands and keep rampant development in check…

After so many posts from the volcanic island of Oahu, you wouldn’t think there was much left, but I couldn’t overlook the most famous volcano of all, although technically Diamond Head (Fig. 1) is a tuff cone like Koko Crater. This brief post is going to examine the internal structure of one of its limbs, on the seaward side.

The lighter color of the rocks in Fig. 2, compared to what we saw at Koko Crater or elsewhere on Oahu, suggests that the underlying magma chamber was depleted of mafic minerals. Dark hues associated with basalt are caused by minerals like plagioclase feldspar, amphibole and pyroxene, and biotite mica. The lighter color of the road cut (fresh and unweathered) suggests that the magma contained felsic minerals like albite and orthoclase feldspar, quartz, and muscovite mica. I could be completely wrong about this but there is no doubt that the rocks in Fig. 2 are not dark gray or black…

My hypothesis is consistent with what is known about the crystallization sequence of minerals from a melt and the resulting viscosity of igneous rocks. Mafic minerals and the lava they form have low viscosity and flow readily, as we’ve all seen in videos of eruptions on the island of Hawaii. These magmas bubble, flow, shoot fire into the air, and release pressure easily. However, felsic minerals (especially quartz) are sticky and have high viscosity, which causes them to resist flow, contain gasses, and eventually explode spectacularly (e.g. Mt. St. Helens).

I think the Diamond Head vent (i.e. volcano) tapped a part of the magma chamber that had already lost most of its mafic minerals, but it wasn’t as explosive as Mount St. Helens.

Another surprising feature I saw along the seaward margin of the Diamond Head tuff cone was a set of vertical joints filled with reddish rock (Fig. 4).

It is important to remember that the entire island of Oahu was constructed by magma escaping through innumerable fissures like those seen in Fig. 4, at first creating thick lava sequences deep beneath the Pacific Ocean’s surface, then flowing through breaks in the jumbled mass of previous flows. By the time the pile of basalt reached the water’s surface to form Oahu, the magma chamber was running out of gas (so to speak), and the lava was thicker and more viscous.

Diamond Head and Koko Craters were the result of these last gasps.

This post concludes my visit to Oahu, an island that rose from the sea less than five million years ago, formed by a huge magma chamber that was created when the Pacific plate slid over an upper mantle hot spot so concentrated that it melted ocean crust an constructed the Hawaiian archipelago, more than 1500 miles long.

I encourage anyone reading this post to explore the amazing story of this new land as it was populated by plants and animals, culminating in the incredible story of how Polynesian culture reached this remote land…

For this post, we went inside Koko Crater (Fig. 1) on the north side (Fig. 2), where the cone was breached, allowing easy access. A road had been constructed and the interior is now filled with a botanical garden and an equestrian center.

Parking is just outside the crater and a trail leads inside (Fig. 3), where a three-mile trail goes around the periphery. We didn’t have time to complete the circuit, so we settled for entering the main crater (see Fig. 2), where the walls were visible but not accessible for close examination (Fig. 4). However, the lower parts that were visible were covered with coarse debris less than 6 inches in diameter. There were some large boulders of vesicular basalt lying around, but they were loose and could have come from anywhere.

The extreme weathering seen on the inner slope in Fig. 4 suggests that the cap rock at least has a different composition, even if it is built from layers of ash. It is important to remember that tuff cones like Koko crater don’t continually erupt for centuries or millennia; they are local phenomena that vent part of the magma chamber that underlies a truly massive volcano like Ko’olau caldera (see Fig. 2). Thus, they are only active for a while, although dating is a problem for such short time scales.

It is important to note that Koko crater as we see it today has been eroded and the interior filled with breccia and ash during and between eruptions. We can’t say how much time passed between the layers seen in the middle of Fig. 5, but it could be hours to weeks. Most tuff cones are active for a couple of months, so the active period of Koko was on the order of a few years. Volcanic vents can produce a lot of ash very quickly.

This is my last post from Koko Crater. I didn’t have time to climb the 1048 steps to its summit, and I’m pretty sure my knees are glad.

In a nutshell, a vent formed along a fracture zone associated with the Ko’olau volcanic system and spewed ash and minimal lava flows onto the surface, where they interacted with the nearby shoreline, all of it lasting only a few decades at most.

Erosion has been minimal so we see Koko pretty much the way Pele left it….

These volcanic rocks were erupted more than 1.7 million years after the devastating collapse of the Ko’olau Caldera. It was probably a last gasp to release pressure within the magma chamber. Exact dating of these younger tuff cones is problematic, but they were all created within about a 50 ky window.

The kinds of volcanic tephra produced by the original Ko’olau volcano and the younger tuff cones shows a tendency towards more ash and less basalt. Certainly, Koko, Diamond Head, and this unnamed crater were part of monogenetic fields. These cones degassed a part of the magma chamber then became dormant; others appeared to perform the same function in another part of the chamber, part of the overall development of the Ko’olau volcano.

With the extinction of the fires here and at Koko Crater, it is safe to say that Ko’olau is dead and the goddess Pele has moved to her new home in the Kilauea volcano, on Hawaii…

This is a quick post to summarize some effects on the beaches of the windward side of Oahu, where basalt rocks of the Ko’olau volcano don’t protect the coast. Kailua isn’t far from Honolulu (Fig. 1) and the climate is similar. The wetter coastline, such as at Nu’uanu park, doesn’t extend this far. Kailua is a broad flat area, unlike the deep valleys of the north shore. The caldera is set back much further from the coast and the basalt is buried.

There is nothing surprising about what we saw at Kailua beach. Beach erosion is ubiquitous around the world; for example, it takes years for scarps like that seen in Fig. 3 to recover from a tropical storm in the Atlantic Basin. Recent studies suggest that in general, sandy beaches are being eroded; the proximate cause is a lack of sediment or increased wave energy, but the root cause most-often blamed is climate change and sea level rise.

We need to stop blaming the climate and reconsider all of the dams and diversions we’ve constructed on rivers that feed the world’s beaches, and ill-considered engineering projects completed in coastal zones.

The beach isn’t a play pool…

Today, we can see some of these rocks up close. Also, these are not as weathered as we saw previously. The lava flows are well preserved at Nu’uanu Pali park (Fig. 4).

This post visited the remains of the Ko’olau volcano, which exploded/collapsed/disappeared into the Pacific Ocean about 1.7 my, leaving a sharp mountain range that was the edge of the central caldera. A much younger tuff cone erupted about 100 thousand-years ago as part of the rejuvenation of the magma chamber. Its last gasp as it were.

This is going to be a relatively short post about some of the older volcanic rocks on Oahu, basalts from the Ko’olau volcano, with an age between 2.8 and 1.7 million years ago (my). I introduced these in a previous post. This post examines some of these rocks up close.

Low rainfall along this coast has brought erosion in the mountains to a standstill. None of the streams that penetrate the Ko’olau Range reach the ocean during the summer and fall. There probably are episodic floods that deliver find-grained sediment from the highly weathered volcanic rocks.

This post has shown some of the characteristics of the Ko’olau Volcano and its associated basalts. The original lava has been chemically altered to produce clay minerals, which are easily transported. There are no mud flats because of the slow weathering and delivery to the coast, although soils can be very rich in the inland parts of Oahu. A previous high-stand of sea level created a bench that is found throughout the island, as noted in a previous post.

We’ll look at what the Ko’olau Volcano looks like today in a later post.