Introduction: Sandy Run is an interesting secondary drainage route as it drains the Chester Valley east end (also known as the Roslyn Valley) in a west-southwest direction, but instead of continuing a short distance further on the Chester Valley floor to join south oriented Wissahickon Creek, it turns in a north direction to flow through a narrow water gap carved between Camp and Fort Hills and then turns in a west and west-southwest direction to join south oriented Wissahickon Creek north of the Chester Valley. North of the water gap Sandy Run is joined by a west-southwest tributary known as Pine Run, which in turn is joined by a southwest oriented tributary known as Little Pine Run. All told the Sandy Run watershed covers an area of about 12.5 square miles, with less than half of that watershed area being upstream from the north oriented water gap.
Figure 1: Topographic Map showing Camp Hill (1), Fort Hill (2), Sandy Run south of Camp Hill (3), The north oriented Sandy Run water gap (4), Pine Run north of Camp Hill (5), Little Pine Run (6), and where Sandy Run joins south oriented Wissahickon Creek (7). The lowland south of Fort and Camp Hills is the east end of the Chester Valley.
Camp and Fort Hills today form a prominent quartzite ridge along the east-northeast edge of the Chester Valley east end and rise 150-200 feet above the early Paleozoic age and limestone-floored Chester Valley to the south and the shale and siltstone-floored Triassic age bedrock region to the north. The Sandy Run water gap is approximately 150 feet deep and is today used by two different railroads and two major highways as a route to cross the high quartzite ridge. The Sandy Run turn to leave the Chester Valley by flowing through a north oriented water gap carved across an erosion resistant quartzite ridge is odd because the prevailing regional drainage direction is in a south direction. Logically Sandy Run should continue in a west or southwest direction on the Chester Valley floor as it flows to reach south oriented Wissahickon Creek. Obviously, the Sandy Run we see today did not erode its water gap and the purpose of this essay is to determine how the water gap was eroded and the present-day Sandy Run drainage basin came into being.
The Sandy Run water gap problem: The Sandy Run water gap provides information that can be used to reconstruct conditions at the time the Sandy Run and Wissahickon drainage basins originated. Rhodes Fairbridge in the Encyclopedia of Geomorphology[i] states water gaps form where a river course established over a preexisting erosion surface has maintained its position while base level has been slowly lowered or broad regional uplift has occurred (antecedence). As a result resistant ridges stand out and major rivers cut across them in narrow transverse valleys. Fairbridge also mentions superposition as a second way water gaps can be formed, which means a stream maintains its position as overlying bedrock that buried an underlying structure (or ridge) is slowly removed.
Figure 2: Detailed topographic map of the Sandy Run water gap area. Sandy Run first flows in a west direction (1) south of Camp Hill before turning in a north direction to flow through the water gap (2) and then to join west-southwest oriented Pine Run (3). The combined flow then moves in a west direction as Sandy Run (4) to join south oriented Wissahickon Creek (5), which eventually reaches the Schuylkill River.
Sandy Run drainage basin bedrock units are early Paleozoic or Triassic in age and provide no evidence of geologic events that occurred since the Triassic Period, which almost certainly include multiple tectonic and erosion events. However no evidence exists today to suggest that the Sandy Run drainage route has maintained a position originally established on a bedrock surface that buried the Camp-Fort Hill quartzite ridge. In other words, no evidence suggests the superposition hypothesis explains the water gap formation. Likewise, the antecedence hypothesis also has serious problems. Sandy Run today is too small to be called a river, much less a major river, but assuming tectonic uplift or lowering does describe how the Sandy Run water gap formed it implies the Sandy Run water gap was eroded by north oriented flow in what otherwise must have been a south oriented drainage region. Perhaps even more serious is the implication that tectonic uplift or lowering somehow altered base level so as to make the resistant ridges stand above the surrounding surface. The Sandy Run drainage basin is so small it is difficult to imagine such a tectonic movement affecting only the narrow quartzite ridge.
Solving the Sandy Run water gap problem: If antecedence and superposition do not explain the Sandy Run water gap then how was the water gap eroded? Imagine the regional landscape before the Wissahickon Creek and its tributary Sandy Run drainage basin were eroded. Based on evidence seen in figures 1 and 2, the entire region at that time was at least as high as the highest points on Camp Hill (and perhaps higher if evidence further downstream in the Wissahickon drainage basin is also considered). Bedrock and geologic structures underlying that higher topographic surface were probably the same as seen on the present day topographic surface—the major differences were the regional drainage was completely different from what it is today and whatever streams existed were flowing on a topographic surface that no longer exists.
At some point, probably fairly recent in terms of geologic time, the drainage system on that high level topographic surface was overwhelmed by gigantic southwest oriented floods. The floodwater source cannot be determined from the Sandy Run drainage basin evidence, but was probably the melting of a large North American ice sheet. Whatever the flood source, immense quantities of southwest oriented floodwaters flowed across all of southeast Pennsylvania and in the region of interest to this essay were first captured by headward erosion of the southeast oriented Schuylkill River valley with floodwaters then moving in a south direction towards the actively eroding south oriented Wissahickon valley and in a west-southwest direction along the present day Chester Valley to reach the newly eroded southeast oriented Schuylkill River valley.
Wissahickon Creek valley headward erosion eventually cut across the present day Chester Valley and ended southwest oriented flood flow through the Chester Valley to the southeast oriented Schuylkill River valley (floodwaters moving on both sides of present day Camp and Fort Hills were captured by the newly eroded Wissahickon Creek valley). At the same time southwest oriented flood flow moving on what are today the Pine Run and Little Pine Run alignments (initially on a higher level surface) continued in a west and southwest direction north of Fort Hill to join south oriented Wissahickon Creek, but also spilled directly south at the Sandy Run water gap location between present day Camp and Fort Hills and joined southwest and west-southwest oriented flood flow moving along the present day west-southwest oriented Sandy Run alignment. As topography both south and north of the present day Camp-Fort Hill ridge was lowered this south oriented flow between Camp and Fort Hills became concentrated and carved a south oriented water gap where the north oriented Sandy Run water gap now exists.
If the Sandy Run water gap was eroded by south oriented water why does Sandy Run flow in a north direction through the water gap today? The immense southwest oriented floods were captured by headward erosion of south oriented valleys from the deep southwest oriented Delaware River valley (also eroding headward along a major southwest oriented flood flow route). In the area of concern for this essay Schuylkill valley headward erosion first captured the flood flow. Next Wissahickon valley headward erosion captured flood flow and in sequence from south to north beheaded flood flow routes to the newly eroded Schuylkill River valley. Further east Pennypack Creek valley headward erosion next captured the flood flow and again in sequence from south to north beheaded flood flow routes to the newly eroded Wissahickon valley. In other words flood flow across the Sandy Run drainage basin ended while further north flood flow continued to reach the newly eroded Wissahickon valley.
Figure 3: Regional map showing south oriented Wissahickon Creek (1), south oriented Pennypack Creek (2), west-southwest oriented Sandy Run south of Camp Hill (3), west-southwest oriented Pine Run north of Camp Hill (4), and the Sandy Run water gap (5).
Pennypack Creek headward erosion beheaded all flood flow to the Sandy Run drainage basin with the result that Sandy Run and its tributaries, Pine and Little Pine Runs, became the minor streams we see today. Flow through Sandy Run water gap diminished and erosion of the water gap almost ceased. The former west-southwest oriented flood flow route north of Fort Hill to the Wissahickon Creek valley captured the greatly diminished flow in Pine and Little Pine Runs and diverted that water directly to the south oriented Wissahickon Creek valley, which in turn reversed flow in the Sandy Run water gap and captured the present day west-southwest oriented Sandy Run headwaters south of Camp Hill. These captures reversed flow through the Sandy Run water gap and created the north oriented Sandy Run water gap and the Sandy Run drainage system we see today.
[i] Fairbridge, Rhodes W., 1968, The Encyclopedia of Geomorphology: Encyclopedia of the Earth Sciences Series, Volume III, Reinhold Book Company, New York. P. 1220.