ROCKWELL FORMATION

Site Location: 

Sideling Hill Aqueduct, mileage 136.56

         Nearest Access: Pearre/Lock 56, mileage 136.20

  The Rockwell, exposed here, along with the Purslane Formation comprise what is referred to in other areas as the Pocono Formation.  It is an assemblage of light colored clastics that include massive, coarse grained to conglomeratic layers, becoming finer grained upwards to form interbedded siltstones and thin sandstones.  In some areas, the top most layers include carbonaceous, black shales that represent the margins of episodic swamp environments in which coal formed.  The Rockwell is not very well exposed along the Canal.  However, it can be fully appreciated in the deep highway cut through Sideling Hill on route I-68 where the State of Maryland has built a Visitor’s Center.

  The Rockwell/Purslane together create a formidable mountain ridge-capping rock.  Sideling Hill is interesting in that it is structurally a syncline.  That means that the rock has been folded into a trench-like configuration as is clearly evident at the I-68 cut.  This rock formation is mechanically very strong and erosion resistant.  It had a profound effect on the Potomac River.  The beautiful meanders of the river (the Paw Paw ‘bends’, Mile Markers 138-157) are constrained between Town Hill ridge on the northwest and Sideling Hill on the southeast, a virtual river wave guide that caused the river to whip back and forth like the coils of a trapped serpent.  At one time, the Potomac River flowed around the east side of what is now the town of Paw Paw, WV, and just upstream of Paw Paw on the opposite side of the river is an abandoned meander channel that now is comprised of Reckley Flat and Purslane Run.  These abandoned meanders are called ‘ox bows’ and are especially prevalent along the Mississippi River.

  The Rockwell is highly fossiliferous, containing specimens of a wide variety of marine species including trilobites, ostracods, crinoids, but clearly dominated by brachiopods, pelecypods, and gastropods (the shelled creatures that inhabit shorelines).  Where black shales are encountered, plant spores are abundant, evidence of a swamp vegetation.

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