BRALLIER-SHERR FORMATION

Site Locations (3): 

Devil’s Alley Camp, mileage 144.50

               (Nearest Access: Fifteen Mile Creek Aqueduct, mileage 140.90)

 Sorrel Campground, mileage 154.0

               (Nearest Access: Outdoor Club Road, mileage 153.40)

 Paw Paw Tunnel, mileage 155.20 to 155.78

               (Nearest Access: Paw Paw Tunnel parking lot, mileage 156.20)

  This thick assemblage of green to olive brown siltstones containing a few thin sandstones is referred to simply as the Brallier in most regions of the eastern U.S.  Here the name Sherr has been added in recognition of a location where this formation has been very carefully studied and described, the small community of Sherr, West Virginia.  Such a location is called a ‘type locality’ which means that it is established as the standard to which rock strata from other locations can be compared.  Variations, generally minor, from place to place will exist due to geographic differences in the depositional environment, for example, the extent to which sand is present compared to the adjoining Foreknobs Formation.  The Brallier is a gradational continuation of the environment in which the Mahantango was deposited and forecasts a coarsening of the sediments transported to the basin, creating the Foreknobs Formation.        

  Devils Alley

Whether it is the Brallier or the Foreknobs, the rocks that are exposed along the rather lengthy segment of the Canal from the north portal of the tunnel through Devil’s Alley (about 12 miles) is most interesting and scenic.  During the last collision of the continental plates, called the Alleghanian orogeny, the rock formations were folded and faulted and transported westward above major thrust faults active in the deep subsurface.  The charm of this Canal segment is the sequence of folds that exhibit second and even third order flexures, all having been formed as part of a broad anticlinal structure.

Another interesting exposure of the Brallier is along the boardwalk leading to the north portal of the tunnel.  Here great slabs of rock slope down to the walkway. (photo below)  The surfaces that are observed are called bedding planes which means that each of them represents exactly the same depositional time over its entire surface.  Of course, the slabs were horizontal or nearly so during deposition.  Often depositional features may be observed such as ripple marks that preserve for all time the rippling action of shallow water that can be observed today in sheltered, shallow pools of the Potomac River.  During the plate collisions that tilted these slabs, they slipped past each other.  The slippage forced them to rub or grate against one another creating polished areas, nearly mirror finished, that clearly establish the direction of the movement.  These are called slickensides.  Gouges may also be observed that were formed in a similar manner.

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