Washington Monument State Park, MD: Familiar Cambrian Metasediments

Figure 1. Looking west from Washington Monument, atop the Blue Ridge in Maryland. The valley is equivalent to the Shenandoah Valley in VA (see Fig. 2), but I couldn’t find a map with it labeled. The Appalachian trail follows the ridge through MD; we encountered it a few miles south of here in a previous post. We expect to see some of the same Proterozoic-to-Cambrian (2500-500 Ma) metasedimentary rocks here that we saw before, in addition to a surprise from an older post.

Figure 2. The field site. Washington Monument is indicated by the purple circle and arrow in the large map. The first inset map shows the geology around the monument. Note the mismatch in geology from different quadrangles; this must be a problem with either the data or Rock D, but the units (indicated in the smaller inset map to the right) were consistent when I clicked on a point. My home is indicated by the star, so you can see we haven’t traveled far. ATWC refers to the Appalachian trail at Weverton Cliff, MD, which I recommend you read to get some background. BRNP represents Bull Run Nature Preserve, which I posted last year. The geological legend for the detailed inset map is at the bottom of the figure. Note that Ma stands for a radiometric age of one-million years; this age is indicative of cooling below the threshold to set the atomic clocks within the minerals, but sedimentary rocks can’t be dated this way. Therefore, these are dates when deep burial and/tectonic deformation/magmatism ceased (i.e. when an orogenic period ended).

Figure 3. Rubble near the monument that resulted from in-place weathering of Weverton Formation rocks (Cw1 and Cw2 in Fig. 2). All of the weathering products (e.g. clays and carbonates) have been washed away, leaving large slabs (~6 feet) piled up. This is a common feature of rocky knolls with good drainage.

Figure 4. (A) Outcrop of older Weverton formation rocks (Cw1 in Fig. 1), revealing weathered material below and boulders on top. This outcrop contains cross-bedded layers on close examination. (B) Photo of a block of Cw1 used in the monument , which shows the crossbedding better than panel A because a fresh surface was cleaved during a recent repair of the 30-foot tower. The color is important: green sedimentary rocks like these represent marine environments, where there is less oxygen; sedimentary rocks deposited in rivers tend to be reddish because of oxidation (rusting) of Fe-containing minerals. These are probably shallow marine sands.

Figure 5. (A) Quartz in a vein (<1 inch thick) from near the monument. Note that the cross-bedding is very similar to Fig. 4 but more weathered. (B) Less-common view of a quartz vein seen obliquely, showing the surface that was against the country rock. These veins would have been injected during a period of magmatism, sometime between 2500 and 511 Ma; I can’t be more specific because I don’t know exactly where the radiometric ages were measured within these rocks. However, the Weverton formation is approximately 4500 feet (1.4 km) thick here; thus it’s possible that these rocks were deposited episodically during this immense time interval; but no unconformity (i.e. erosion or non deposition) is mentioned in RockD.

Figure 6. View looking east from the Appalachian Trail, showing the terrain typical of the Appalachian foothills. To the left of center, outcrops of Weverton rocks (Cw1 and Cw2 in Fig. 2) can be seen.

Figure 7. (A) Boulder (~2 feet across) of arkose, revealing angular clasts of rock fragments in a sandy matrix. (B) Poor outcrop of conglomerate with rounded rock and quartz in a similar, sandy matrix. Comparing these images to Fig. 4 shows the variability of sedimentation (and thus depositional environment) during relatively short time intervals (say … tens of millions of years, for example). This kind of variability implies changing sediment sources, possibly caused by tectonic uplift (with magmatism) to the east.

Figure 8. This figure is from the Bull Run Nature Preserve field trip. It is a schematic of how layers of sedimentary rocks (shown in different colors) can slide over one another along thrust faults. This process results in stacking of similar sediments, making stratigraphic analysis of sparse field data problematic. The rocks on the left are sliding upward to the right along a series of thrust faults (dashed line). At Bull Run Nature Preserve, a fault like this could be identified by older rocks clearly being stratigraphically higher than younger ones. That isn’t the case at Washington monument, where the interleaved rocks (blue and green) are too similar in lithology and age to be differentiated.

SUMMARY

The thrust fault labeled in Fig. 2 has been confidently identified (represented by a solid line), no doubt through more investigation than I was willing to spend time on. This unnamed fault underlies the northern Blue Ridge, and marks the beginning of the Valley and Ridge province; the Blue Ridge was thus an anomaly, which has been identified as a belt of older rocks thrust over younger ones about 500 million-years ago, when the supercontinent of Pangea was being created.

We have followed the Weverton formation through time (2500-485 Ma) and space (more than 40 miles). During this unimaginable interval, this small piece of the Earth’s crust has moved thousands of miles. Only the last 500 my of its journey is known with any confidence. This tectonic plate has been carrying these sediments to unknowable latitudes, colliding with immovable objects while spreading the remnants of mountain ranges that are now forgotten, deconstructed by the irresistible power of water, wind, ice and time.

Some things aren’t meant for us to know …