Deborah Painter (USA)
In many states of the United States and in many locales in the United Kingdom, there are historic markers at the site of an important historic home or event. However, I wonder if every accessible rock formation had its own historic marker, would more people take the time to learn about it? The entire history of the planet is seen in rock formations.
Just west of the town of Hancock, in the state of Maryland, USA at Mile Marker 74 on Interstate 68 (coordinates 39° 43’ 11.54” N, 78° 16’ 58.29” W) is the Sideling Hill road cut, a textbook example of tight folds in a mountain (Fig. 1). Until relatively recently, the visitors centre located adjacent to the cut was a perfectly complete historic marker. It gave travellers not only a place to stop to buy refreshments and relax at a picnic table surrounded by shade trees. It also provided an opportunity to read about the history of a spectacular cut in a mountain resulting from a need for safer transportation through a difficult and rugged stretch of road.
The centre still helps motorists see a geological formation safely from a walkway and an enclosed bridge. Sideling Hill’s transportation story goes back to the earlier days of road travel in Maryland’s mountains on the old National Road. It was a frightening drive for those not accustomed to steep roads and downward curves. Drivers who wanted to avoid this segment had to go out of their way to get from this narrowest point in Maryland’s mountainous “panhandle” to West Virginia, or east to larger population centres. Something had to be done and it was going to be expensive. There was a railway tunnel through the mountain some distance away.
A similar road tunnel was considered as an alternate to blasting the mountain itself. However, this idea was abandoned by the Maryland Department of Transportation as not practical for assisting motorists needing emergency services, especially if medical transport was required.
As a result, Sideling Hill’s summit was sliced in half in an operation that began in 1985 and continued until 1991, when the section of the Interstate was opened to the public. With the use of 2,641 metric tons of explosives, approximately 90,718 metric tons of shale, sandstone, coal and conglomerate were blasted away, revealing the inner structure of the mountain. The surface elevation at the top of the ridge is approximately 495m. Along both flanks of the road cut are ledges designed to catch falling rocks, which continue to fall and constitute a potential threat.
Folding is one of the most intriguing geological phenomena because, for many generations of geologists, the cause for most folds was mysterious. The immense weight of eroded sediments causing isostatic uplift (the geosyncline concept) was the most cited cause until the mid-1960s. At this time, an understanding of plate tectonics showed the true mechanism of folding and mountain building (“orogenies”). As a result, the concept of geosynclines forming folded mountains is now obsolete.
Folding is usually seen in sedimentary layers that accumulated and became rock along the margins of plates. In the crust where rocks are cool and relatively brittle, folding can take place if the stress of tectonic activity is constant and gives the rock enough time to bend.
Sideling Hill lies in the ‘Valley and Ridge Physiographic Province’ of eastern North America. The “hill” is a syncline mountain between two anticlinal valleys (see below for an explanation and Fig.2). The mountain is approximately 160km in total extent as it trends from its south-western end in Morgan County, West Virginia to Washington County, Maryland and northeast into the Warfordsburg area of Fulton County, Pennsylvania, where it increases in elevation to its highest point. Laurel Ridge and Chestnut Ridge to the west of Sideling Hill are part of this complex of tightly folded ridges.
A syncline is a particular sort of fold in which the strata on either side dip inward toward the axis of the fold – that is, the fold is seen to go downwards in the middle. Such a feature is very dramatically seen at Sideling Hill. It is a wonderful illustration of how synclines or downfolds do not necessarily form valleys, and anticlines, or upfolds, do not necessarily form hills or ridges. The location of hills and valleys is often less a function of the topographic highs and lows of the folds, and more a function of where the more erosion resistant rocks are within the folds.
In the case of the folded mountains of western Maryland, compression stresses built up in the crust as a result of the collision of two plates, one bearing the present day African continental rock and the other bearing the present day North American continental rock. Geologists have named this mountain building event the Alleghenian Orogeny. It reached its peak during the late Permian or early Triassic, approximately 230 to 240 million years BCE.
The Alleghenian Orogeny contributed to the formation of the supercontinent Pangea. Not since the ancient Proterozoic supercontinent of Rodinia had all the continents come together in this way. To the east of Sideling Hill, the great mountain range was glaciated on its summits.
At Sideling Hill, the layers are shale and sandstone. They formed much earlier than the Alleghenian Orogeny that folded them. The Purslane Formation is the series of layers at an elevation above the Rockwell Formation seen in Fig. 3.
Both are of Mississippian (Lower Carboniferous) age. This time period lasted from approximately 330 to 345 million years BCE. The Lower Carboniferous subperiod of Europe corresponds to the Mississippian period of North America (with the Upper Carboniferous being referred to as the Pennsylvanian in the USA). The Purslane Formation consists of distinct beds of relatively resistant conglomerates and sandstones, with interbedded coaly shales in which the occasional fossil plants occur. They are the fascinating scale trees of the Order Lepidodendrales, titanic relatives of our modern small clubmosses and, of course, long extinct (Fig. 4).
Coal is relatively uncommon in Lower Carboniferous subperiod deposits in North America, becoming much more common in the Upper Carboniferous. No marine fossils have been found to date in the Purslane Formation, which provides a good clue that the elevation of the sediments was well above sea level. The environment was probably a vast floodplain and predominantly freshwater.
What we see at Sideling Hill represents only 106m of the Formation as known elsewhere. It is expected that many metres of this Formation have been erased by erosion since the Triassic period and it is also likely we will never know how much was present initially. When we see many distinct beds, we can surmise that there were times when the stream channel shifted.
The Rockwell Formation is comprised of less resistant black silty shale, reddish brown claystone, clay rich sandstones, and coaly shales. The Rockwell Formation and the equally less resistant Hampshire Formation make up the valleys between this ridge and others to the west and east.
The Rockwell Formation was likely a floodplain near and at sea level during Lower Carboniferous time. It features a combination of scale tree fossils and marine invertebrate fossils in the black silty shale that one can observe 51m to 54m above the lowest exposed strata at the cut at Sideling Hill. These fossils consist of bivalves, mainly of the genera Wilkingia and Aviculopecten (Fig. 5) and brachiopods, represented by a very large number of genera. Terrestrial wetland species preserved as fossils here include imprints of plants and fragmentary wetland plant fossils, generally of the genus Lepidodendron and others of the order Lepidodendrales.
According to David K Brezenski, whose 1989 paper on Maryland’s Mississippian strata is listed in the references section of this article, the Riddlesburg Shale is a correlative elsewhere in the adjacent state of Pennsylvania, in the central portion of the state. This Formation recalls a time when the shoreline shifted, submerging an area from what is now eastern Ohio to what is now western Maryland.
The Sideling Hill road cut is wet during much of the year, since rainwater percolates from the crest down to a resistant clay layer and emerges from the cut in the same manner as groundwater emerging from a cliffside. In winter, when this rainwater freezes, the result is rather spectacular.
Visiting the site
Sadly, due to Maryland budget reductions, the visitors centre with brochures, snack machines, and full time staff, has been closed for a number of years and its contents moved to nearby Hancock to its visitors centre, located at 326 East Main Street, Hancock, Maryland, 21750. The telephone numbers are (301) 678-5463 and (301) 678-6035.
You can still relax at the picnic tables on site, use the restrooms, view the Vietnam Veterans Memorial, and walk across the Victor Cushwa Memorial Bridge, an almost completely fenced-in pedestrian walkway spanning the Interstate. You can also walk up the pedestrian walkway behind the visitors centre building to view the north face of the road cut in more detail (Fig. 6). Leaving the elevated pedestrian walkway near the road cut to climb the rocks for closer inspection is not permitted.
It is a fairly long drop down from the walkway and a longer drop down the slope to the highway. From the picnic area you can see Pennsylvania (Fig. 7).
More information on the amenities mentioned above is available at the following web site: Sideling Hill Welcome Center | VisitMaryland.org.
About the author
Deborah Painter is an ecologist and general environmental scientist specialising in transportation and industrial development planning to minimise deleterious environmental impacts. She lives in the United States.
Bjerstedt, T.W., 1986. Regional stratigraphy and sedimentology of the Lower Mississippian Rockwell Formation and Purslane Sandstone based on the new Sideling Hill road cut, Maryland: Southeastern Geology, v. 27, p. 69-94.
Brezinski, D.K., 1989. The Mississippian System in Maryland: Maryland Geol. Survey Report of Investigation 52, 75 p.
Conkwright, Bob. 2005; electronic version created 2021. Geology of the Sideling Hill Road Cut. Maryland Geologic Survey, Division of Coastal and Estuarine Geology.
Coskren, T. D. November 15, 2019. Sideling Hill Geology Field Trip. Montgomery College instructional video.
Kazlas, M. Alan. 2021. Palaeos Life Through Deep Time.
Pelletier, B.R., 1958.Pocono paleocurrents in Pennsylvania and Maryland: Geological Society of America Bulletin, v. 69, p. 1033-1064.
Read, C.B., 1955.Floras of the Pocono Formation and Price Sandstone in parts of Pennsylvania, Maryland, West Virginia, and Virginia: U.S. Geological Survey Professional Paper 263, 32 p.
Reger, D.B., 1927. Pocono stratigraphy in the Broadtop Basin of Pennsylvania: Geological Society of America Bulletin, v. 56, p. 397-410.