THE ANATOMY OF A ROCK SLIDE

The Sherando Lake rockslide is more than just a rockslide. In analyzing the rockslide and its causes, a number of discoveries were made. It contains Skolithos liniaris (worm tubes), which are about 500 million years old. This makes these fossils the oldest found in Virginia. It may also contain evidence of Sea Pens, crinoids that occurred in the early Cambrian. Other fossil traces like mud cracks and bioturbation activity can also be seen.

From the information collected and through the work of others, it is possible to put together the history of the rock at Sherando Lake. The quartzites began life as beach sandstone and were deposited in a barrier island complex—a nearshore to offshore shallow marine environment with pure quartz sand, similar to what we find today in the Outer Banks in North Carolina. Based on large numbers of worm tubes and other signs, it was teeming with life.

It then was buried, metamorphosed and thrust about 100 miles to where it exists today. Quartzites are extremely resistant to erosion. However, in more geologically recent times, the colder climate has resulted in ice wedging along fractures in the rock.

The anatomy of the rockslide indicates that the lithic composition of the slide is homogeneous; however, significant changes in the rocks’ appearance were found in some places. The quartz in the sandstone is well cemented with silica but in other places of the slide it is fine grained, laminated and layered. These changes are noticeable in the higher points of the slide where the quartzite sandstone is well structured, while in the places near the vegetative border, rocks with more layers appeared. The lack of vegetation in parts of the slide suggests that it is still moving. The relative homogenous size of rocks indicates a uniform fracture pattern.

In summary, the Sherando Lake rockslide is moving today and is the result of ice wedging. It contains the oldest fossils in Virginia. To fully unravel all of its mysteries will require additional study.