Only about 350 feet of the formation occur on Sideling Hill, the remainder, an unknown thickness, having been removed by erosion. Overlying the Rockwell Formation is the Purslane Formation, typified by gray-green, tan, and white, cross-bedded sandstones and quartz-pebble conglomerates with interbedded gray siltstones, shales, and coaly shales. Post mountain-building erosion to the ridges and valleys seen today. Folding during the Alleghenian mountain-building episode. Shallow marine waters and adjacent shoreline swamps of the Riddlesburg sea. Sequence of development of the rocks exposed at Sideling Hill. Sequence of development of the rocks exposed at Sideling Hill.įIGURE 3. The Riddlesburg Shale records a major shift of the shoreline which submerged an area from eastern Ohio to western Maryland (Figure 3A). The Rockwell Formation in the area of Sideling Hill was probably deposited in an alluvial plain environment near sea-level. The Sideling Hill exposure is the first recognition of this zone in Maryland. A similar marine unit has been recognized in correlative rocks in central Pennsylvania, where it has been termed the Riddlesburg Shale. The fossils are generally rare within these intervals and consist of brachiopods and bivalves. Marine fossils are present within the black silty shale 165 to 178 feet above the lowest exposed strata (B in Figure 2).
The origin of such diamictites is highly debatable and no generally accepted theory has yet been proposed.įossils are moderately common in the Rockwell Formation, but almost all are plant fragments and imprints. The larger pebbles and cobbles consist of a multitude of lithologies including granite, graywacke, chert, and quartzite. A diamictite is a very poorly sorted to unsorted rock composed of clay, silt, sand, and pebbles or cobbles. An even rarer and indeed unusual lithology, termed diamictite, is present approximately 70 feet above the base of the section (A of Figure 2). These coals are interesting in that coal is typically not common in Lower Mississippian strata. In places, thin shaly coals and coaly shales are interbedded with shales and siltstones. At the road cut, approximately 450 feet of the Rockwell Formation are exposed and consist of interbedded, tan and gray-green, clay rich sandstones, gray-green to dark-gray, silty shales, and gray to dark-gray, sandy siltstones with several intervals of red-brown claystone near the top. The Rockwell and Purslane Formations were deposited during the early Mississippian, about 330 to 345 million years ago. Between these two ridges the intervening lower area is composed predominantly of Devonian age shales and siltstones. This topographic inversion, in which the structural low becomes a topographic high, is also seen at Town Hill, the next major ridge to the west and a structural twin to Sideling Hill. The valleys on either side are underlain by more easily eroded rocks of the Rockwell and Hampshire Formations. Geologic cross-sections of the north (top) and south (bottom) sides of the I-68 road However, the youngest rocks, or those highest in the stratigraphic section, are the resistant sandstones and conglomerates of the Purslane Formation, which occur in the center of the fold and cap the ridge.įIGURE 2.
At first, Sideling Hill may appear to be a somewhat unusual feature, inasmuch as the downfold, or syncline, exposed in the road cut would seem to be more likely to form a valley, rather than a ridge.
THE CUT SERIES
Erosion of these folds has produced a series of subparallel ridges and valleys, in which the ridges are capped by erosion-resistant sandstones, and the intervening valleys are underlain by soluble limestones and easily eroded shales. Sideling Hill lies in the Valley and Ridge Physiographic Province of eastern North America, a region characterized by tightly folded strata. This exposure is an excellent outdoor classroom where students of geology can observe and examine various sedimentary rock types, structural features, and geomorphic relationships. Although other exposures may surpass Sideling Hill in either thickness of exposed strata or in quality of geologic structure, few can equal its combination of both. Almost 810 feet of strata in a tightly folded syncline are exposed in this road cut.
One of the best rock exposures in Maryland and indeed in the entire northeastern United States is located approximately 6 miles west of Hancock in Washington County, where Interstate 68 cuts through Sideling Hill (Figure 1).
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