Chester Valley Origin: Some Alternate Interpretations


To date this website has used simple map interpretation methods when interpreting Philadelphia area landform origins and previous researcher interpretations have not been mentioned. While multiple Philadelphia area geologic and geographic studies have been conducted over the past 150 years origins of most landforms interpreted here have never been addressed at least in a way that has produced much agreement. Much debate has focused on larger-scale Appalachian landforms while similar Piedmont region landforms have been ignored. For that reason there is little or no published research interpreting many specific Philadelphia area landform features. However, there are exceptions and the Chester Valley is such an exception. The purpose of this essay is to briefly illustrate how previous investigators have attempted to explain the Chester Valley origin.

Topographic and geologic maps of the Chester Valley are included in the 1908 United States Geological Survey Philadelphia folio[1]. These topographic and geologic maps and the accompanying discussion provided enough topographic and geologic data to stimulate further interest. Nevin Fenneman in his 1938 volume Physiography of the Eastern United States, states the Chester Valley “begins at Quarryville, south of Mine Ridge, and extends 55 miles east-northeast in a strikingly straight course to a point north of Philadelphia. Throughout its length it constitutes a trench 1 to 3 miles wide, and several hundred feet below the nearby uplands. As this area has been repeatedly peneplaned, the synclinal structure of the valley can only affect its form indirectly, by causing a zone of soluble limestone to be preserved between slates, quartzites, and schists on either side. Its present surface is part of the youngest peneplain. There is no evidence that this valley was ever followed by a stream. Not only the Schuylkill River, but smaller creeks cross its course with no regard for its presence, their courses conforming to the general slope of the province and perhaps determined by superposition.[2]

Twenty-seven years later Thornbury in his Regional Geomorphology of the United States describes the Chester Valley as being “a remarkable limestone valley that lies along the north edge of the Piedmont Upland in southeastern Pennsylvania and extends from Quarryville northward for 55 miles to the Schuylkill River north of Philadelphia. The valley is coextensive with the outcrop area of the Ordovician Conestoga limestone. It is from 1 to 3 miles wide, is unusually straight, and lies several hundred feet below the uplands to either side. The south side of the valley is formed by the Wissahickon schist member of the Glenarm series, and the north wall of the valley by the Lower Cambrian Chickies quartzite. There is no evidence to suggest that a stream ever followed the valley; in fact, all streams that enter it the valley cross it rather than flow along it. Explanation of this unusual valley is linked with whatever interpretation is given to the so-called “Marctic Overthrust.”[3]

Thornbury goes on to discuss the Marctic overthrust problem. “In 1929, Knopf and Jonas described what they considered a thrust that was supposed to have carried the Wissahickon schist of presumed Precambrian age northwestward for a distance of 20 miles onto the Conestoga limestone of Ordovician age. Thus the Chester Valley south wall was considered the eroded front of the low-angle Marctic overthrust sheet. Some geologists deny the existence of the Marctic overthrust. Those who contend that the Wissahickon schist of the Glenarm series rests upon a Cambro-Ordovician sequence in normal stratigraphic succession (Mackin, 1935[4]; Cloos and Hietanen, 1941[5])…”

Focusing on the problems this website is trying to address Thornbury adds, “Explanation of the unusual topographic characteristics of the Chester Valley presents a problem as puzzling as the stratigraphic relationships along it. Knopf and Jonas[6] would interpret it as a window in the Marctic thrust. Mackin contended, however, that the walls of the valley are too straight for a low-angle thrust contact, which is usually irregular or sinuous in plan. He believed that the topography along the south side of the Chester Valley suggested more a high-angle fault contact. Bricker and associates (1960)[7] concluded that the valley might be explained as the result of more rapid weathering of a calcareous unit in the stratigraphic sequence. It would be unusual indeed to find a valley with the dimensions and striking linear form of Chester Valley that developed this way. Innumerable limestone valleys exist in the Appalachians whose axes follow the strike of the beds responsible for them, but they either have streams flowing along them or display evidence that streams at one time did flow through them. Lack of evidence of a former longitudinal stream in the Chester Valley makes it seem more likely that it is a product of faulting, whether this be overthrust faulting as postulated by Knopf and Jonas or normal faulting as suggested by Mackin.”

As can be seen by the above quotes from the Fenneman and Thornbury books the Chester Valley unusual topographic characteristics have been recognized by previous researchers who have proposed a variety of different hypotheses to explain the evidence. Further, while earlier researchers proposed hypotheses to explain the Chester Valley evidence there was no agreement among the early researchers as to which hypothesis might be preferred. And finally none of the early researchers considered the possibility of massive and prolonged southwest oriented floods, which is the hypothesis being tested with the evidence illustrated on this website. Entries on this website describing Chester Valley and related evidence include the following:

Water Gaps, Wind Gaps, and Through Valleys in and near Valley Forge National Historical Park

The Chester Valley Drainage Divide between the Delaware and Schuylkill Rivers

Water and Wind Gaps near Christiana, Pennsylvania

Chester Valley between Coatesville and Downington

Chester Valley between Buck Run and West Branch Brandywine Creek

Chester Valley between East Brandywine Creek and Valley Creek

Numerous other entries further address the Chester Valley and its east-northeast extension (the Whitemarsh Valley) or the rivers and streams flowing across the Chester and Whitemarsh Valleys.

[1] Bascom, F., W.B. Clark, N.H. Darton, G.N. Knapp, H.B. Kummel, and R.D. Salisbury, 1909, Philadelphia folio, Norristown, Germantown, Chester, Philadelphia, Pennsylvania-New Jersey-Delaware Folios of the Geologic Atlas 162, United States Geological Survey, Washington, DC.

[2] Fenneman, Nevin, 1938, Physiography of the Eastern United States, McGraw-Hill Book Company, Inc., New York, p. 154.

[3] Thornbury, William D., 1965, Regional Geomorphology of the United States, John Wiley and Sons, Inc. New York, p. 96.

[4] Mackin, J.H., 1935, The problem of the Marctic overthrust and the age of the Glenarm series in southeastern Pennsylvania, Journal of Geology, v. 43, p. 356-380.

[5] Cloos, Ernst, and Anna Hietanen, 1941, Geology of the “Marctic overthrust” and the Glenarm series in Pennsylvania and Maryland, Geological Society of America Special Paper 35, 207 pp.

[6] Knopf, E.B., and A.I. Jonas, 1929, Geology of the McFalls Ferry-Quarryville district, Pennsylvania, United States Geological Survey Bulletin 799, 189 pp.

[7] Bricker, O.P, C.A. Hopson, M.E Kauffman, D.M. Lapham, D.B. McLaughlin, and D.U. Wise, 1960, Some tectonic and structural problems along the Susquehanna River, Guidebook 25th Annual Field Conference, Pennsylvania Geologists, 99 pp.

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