Origin of the north oriented Valley Creek water gap between Mount Joy and Mount Misery, Valley Forge National Historical Park, Pennsylvania

Authors

Valley Creek originates as a north oriented stream along the Chester Valley south wall and turns in an east-northeast direction to flow along the Chester Valley floor, but before reaching the southeast oriented Schuylkill River turns in a north direction to cross the Chester Valley north wall between Mount Joy and Mount Misery and to join the Schuylkill River as a barbed tributary. The north oriented water gap between Mount Joy and Mount Misery is located along the Montgomery-Chester County line in Valley Forge National Historical Park and is unusual because Valley Creek turns from the east-northeast oriented Chester Valley to flow in a north direction to join what is primarily a southeast oriented river as it crosses erosion resistant bedrock forming the Chester Valley north wall. With very few exceptions streams flowing through other regional water gaps are oriented in south directions. Why does Valley Creek leave the Chester Valley to flow through a north oriented water gap to reach a southeast oriented river?

1detMtJoywatergapFigure 1: North oriented Valley Creek water gap between Mount Joy and Mount Misery in Valley Forge National Historical Park. Note how the Schuylkill River turns from flowing in a southeast direction to flow in a northeast direction at the point where Valley Creek joins it. The east-northeast oriented Chester Valley is south of the water gap.

Trying to explain the Valley Creek water gap by commonly accepted water gap origin interpretations:

Based on literature reviews Douglass et al in 2009[i] identified four mechanisms previous investigators had proposed to explain transverse drainage or water gap development. These mechanisms are antecedence, superposition, overflow, and piracy. Douglass et al then considered 20 randomly selected southwest United States transverse drainage example sites and determined eight had developed by overflow, seven by superposition, four by antecedence, and one by piracy. The Valley Creek water between Mount Joy and Mount Misery can be studied using the criteria Douglass et al proposed to determine which if any of these four proposed development mechanisms might explain its origin.

Antecedence: Douglass et al state, “For antecedence to take place, a stream must drain across and erode a channel into an uplifting bedrock structure.” Mount Misery and Mount Joy are composed of erosion resistant Cambrian quartzite while bedrock upstream from the water gap is Cambrian dolomite and downstream from the water gap Triassic sandstone, shale, mudstone, and siltstone. While the Valley Creek region experienced tectonic activity, that activity probably occurred during the Paleozoic and Mesozoic and was followed by long periods of deep regional erosion. Antecedence requires uplift of the quartzite ridge after Valley Creek established its course, but all evidence suggests the Valley Creek course is younger than any regional tectonic activity. No evidence supports an interpretation that antecedence explains the Valley Creek water gap origin.

Superposition: Douglass et al state, “Superposition requires that a stream flow atop a cover mass that buries a comparatively resistant bedrock structure.” While the Valley Creek region probably once stood considerably higher than it does today there is no evidence suggesting comparatively less resistant bedrock ever buried the present day Mount Joy-Mount Misery quartzite ridge. What would have stood higher would be composed of rocks similar to those found in the Valley Creek region today. Assuming the region did stand higher it was probably subjected to multiple erosion cycles and the present day Valley Creek course was probably established during the most recent cycle. No evidence supports a superposition origin for the Valley Creek water gap.

Overflow: Douglass et al state, “A river must have been ponded in a lake prior to the formation of an overflow transverse drainage,” which develops when the river spills across a resistant rock structure. The Chester Valley could not have filled with enough water to cross the Mount Joy-Mount Misery quartzite ridge so as to erode the north oriented Valley Creek water gap because the much lower Chester Valley floor extends in a west-southwest direction to the south oriented East Branch Brandywine Creek valley and in an east-northeast direction to the much lower southeast oriented Schuylkill River valley. While overflow from the north might be possible, such overflow would have eroded a south oriented water gap and would not have been possible if the Schuylkill River valley existed. Simple overflow does not explain the Valley Creek water gap.

Stream piracy: Douglass et al state, “The ‘piracy’ or ‘capture’ of a stream occurs when part of a channel’s previous course changes to that of another stream.” The suggestion that headward erosion of the north oriented Valley Creek water gap had captured an east-northeast oriented stream flowing in Chester Valley to reach the Schuylkill River makes no sense as the Chester Valley is much lower in elevation than the Mount Joy-Mount Misery quartzite ridge and is also underlain by easier to erode bedrock. Further, if any stream seen in figure 2 shows evidence of being captured it is the Schuylkill River, which makes a large U-turn just north of the Valley Creek water gap. The Schuylkill River may have captured a south-oriented stream flowing through the water gap, and if so flow in the water gap was reversed from flowing in a south direction to flow in a north direction, which implies that flow in Valley Creek upstream from the water gap was also reversed from a west-southwest direction to an east-northeast direction. Such a capture requires a much more complex origin than anything described by Douglas et al.

2MtJoywatergapFigure 2: Topographic map shows the Valley Creek water gap region. Note how southwest oriented Perkiomen Creek (1) joins the south oriented Schuylkill River (2) in the map north center area and how the combined flow then continues in a southwest, south, and southeast direction to the Valley Creek water gap (3) north end before turning a northeast direction. East of figure 2 the Schuylkill River turns in a southeast direction to eventually join the southwest Delaware River at Philadelphia.

Developing an explanation for a south oriented Valley Creek water gap

As seen in figure 2 north of the water gap southwest oriented Perkiomen Creek joins the Schuylkill River and the combined flow then moves in a southwest, south, and southeast direction to the Valley Creek north end before turning in a northeast direction. While not seen in figure 2, but seen in figure 5 the Schuylkill River is primarily a southeast oriented river flowing to the southwest oriented Delaware River. The joining of two large south oriented drainage systems north of the Valley Creek water gap suggests the possibility that water once flowed in a south direction between Mount Joy and Mount Misery and into the Chester Valley where it then flowed in a west-southwest direction. To test this hypothesis one must look at the region where Valley Creek begins as seen in figure 3.

3TwoValleyCksFigure 3 shows the region where Valley Creek originates (1) as a north oriented stream on the Chester Valley south wall and then flows onto the Chester Valley floor where it turns in an east-northeast direction (2) to eventually turn in a north direction (east of the map) to reach the Schuylkill River by flowing between Mount Joy and Mount Misery. What is interesting about the Valley Creek headwaters is that a second Valley Creek (3) originates on the Chester Valley south wall just west of the first Valley Creek and also flows in a north direction onto the Chester Valley floor, but once on the Chester Valley floor instead of flowing in an east-northeast direction it turns to flow in a west-southwest direction (4) toward the East Branch Brandywine Creek valley.

As seen in figure 3 a second Valley Creek originates on the Chester Valley south wall just west of the first Valley Creek and flows in a north direction adjacent to the first Valley Creek onto the Chester Valley floor where it turns in a west-southwest direction to flow along the Chester Valley floor, but as seen in figure 4 before reaching south oriented East Branch Brandywine Creek the second Valley Creek turns in a south direction to enter a narrow valley carved in the Chester Valley south wall and to join the East Branch Brandywine Creek almost two miles south of the Chester Valley. Presence of this second Valley Creek supports the hypothesis that water once flowed in a south direction between Mount Joy and Mount Misery and then in a west-southwest direction along the Chester Valley floor with the water reaching the south oriented East Branch Brandywine Creek valley and eventually reaching the southwest oriented Delaware River.

4ValleyCkwestFigure 4 illustrates how the second Valley flows in a west-southwest direction on the Chester Valley floor (1) and then turns in a south direction (2) to flow through a narrow valley eroded in the South Valley Hills and to eventually reach the East Branch Brandywine Creek (3) at a point south of the area shown.

Based on present day topography seen in figure 4 the second Valley Creek logically should not turn to the south to reach the East Branch Brandywine Creek, but should instead continue to flow in a west-southwest direction to where the East Branch Brandywine Creek crosses the Chester Valley floor. The fact that the second Valley Creek now turns in a south direction to flow through the South Valley Hills means the second Valley Creek course was established at a time when the Chester Valley floor was at least as high as the tops of the South Valley Hills today. Second the fact that the second Valley Creek turn to flow south through the South Valley Hills is evidence that large volumes of water were involved, with some of the water eroding the south oriented second Valley Creek valley through the South Valley Hills while the remainder of the water lowered the Chester Valley floor elevation and then drained south to help erode the East Branch Brandywine Creek valley, also through the South Valley Hills.

The Perkiomen Creek and Schuylkill River evidence north of the Valley Creek water gap and the second Valley Creek evidence strongly suggest that the water gap between Mount Joy and Mount Misery was eroded by south oriented water coming from either the southwest oriented Perkiomen Creek drainage basin or the southeast oriented Schuylkill River drainage basin. Further, the second Valley Creek evidence strongly suggests erosion of a south oriented water gap between Mount Joy and Mount Misery began when the Chester Valley floor was at least as high as tops of the South Valley Hills, which are comparable in elevation to the present day tops of Mount Joy and Mount Misery. But, today the first Valley Creek flows in an east-northeast direction on the Chester Valley floor and then in a north, not a south, direction between Mount Joy and Mount Misery so further explanation of the water gap origin is required.

Developing an explanation for a reversal of flow in the south oriented water gap between Mount Joy and Mount Misery

The water gap between Mount Joy and Mount Misery was initially eroded by south oriented water, coming from or across the present day Perkiomen Creek drainage basin and/or from further upstream in the present day Schuylkill River drainage basin. West-southwest oriented flow from the water gap south end to the East Branch Brandywine Creek valley also lowered the Chester Valley floor from elevations comparable to hill tops surrounding the Chester Valley today. Such a situation would not have been possible if the present day Schuylkill River valley downstream from Mount Joy had been in place. If so, then how and why was the downstream Schuylkill River valley eroded?

Figure 5 shows how the Schuylkill is a southeast oriented river with some incised meanders and northeast, east-northeast, and south jogs before entering a straight southeast oriented gorge with walls as high as tops of Mount Joy and Mount Misery. While water derived from within the present day Perkiomen Creek drainage basin and/or the upstream Schuylkill River drainage basin over long periods of time could have eroded the water gap between Mount and Joy and Mount Misery such an explanation does not account for the downstream Schuylkill River valley, especially the deep Schuylkill River gorge between Conshohocken and Philadelphia, which must have been eroded at approximately the same time as the water gap was eroded. The southeast oriented Schuylkill River valley must have eroded headward from the Delaware River valley at Philadelphia and have captured the south oriented water moving between Mount Joy and Mount Misery and into the Chester Valley. Headward erosion of this deep southeast oriented valley must have caused a reversal of flow in the water gap between Mount Joy and Mount Misery, which in turn caused a reversal of flow on the Chester Valley floor to create the present day east-northeast and north oriented Valley Creek course and also beheaded west-southwest oriented flow to the second Valley Creek.

5SchuylkillRiverFigure 5 illustrates the Schuylkill River course as it flows from the map northwest corner to the Valley Creek water gap north end (1) and then across the Chester Valley (2) to reach a southeast oriented gorge (3) surrounded by uplands almost as high as Mount Joy and Mount Misery. The east-northeast oriented Valley Creek course on the Chester Valley floor originates west of map area and is identified with the number 4 and the number 5 identifies Perkiomen Creek upstream from where it joins the Schuylkill River.

In summary the Valley Creek water gap between Mount Joy and Mount Misery was initially eroded by immense volumes of south or southwest oriented water that initially flowed on a topographic surface at least as high as the present day tops of Mount Joy and Mount Misery. The water was moving to what at that time was an actively eroding south oriented East Branch Brandywine Creek valley that was eroding headward from the southwest oriented Delaware River valley. At that time the Schuylkill River valley and Chester Valley did not exist and those valleys were eroded by the same water source at approximately the same time. The water source can not be determined from the regional evidence, but probably was glacial melt water that moved in a southwest direction across New Jersey and then across southeast Pennsylvania until captured by headward erosion of a progressive sequence of deep southeast and south oriented valleys such as the Schuylkill River valley. Whatever the water source headward erosion of the deep Schuylkill River valley beheaded south oriented flow between Mount Joy and Mount Misery, which caused a reversal of flow in the water gap and also in the Chester Valley so as to create the present day east-northeast and north oriented Valley Creek course.

[i] Douglass, J.n, Meek, N., Dorn, R., and Schmeeckle, 2009, A criteria-based methodology for determining the mechanism of transverse drainage development, with application to the southwestern United States. Geological Society of America Bulletin; v. 121; no. 3/4; p.586-598.

 

 

 

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