Philadelphia area drainage systems and erosional landforms when viewed on detailed topographic maps can be considered to be pieces of a giant puzzle, which if properly assembled tells how the regional landscape was formed. The topographic maps which can be obtained from the United States Geological Survey National Map and TopoView websites provide a big picture view which cannot be seen from the ground and which most air photo and satellite imagery do not show as well. To date the geology research community has not solved the puzzle, but as this series of essays will demonstrate the puzzle does have a solution, although that solution tells a story very different from the story the geology research community usually tells.  

To recognize the map drainage system and erosional landform evidence as puzzle pieces it is necessary to identify the relevant pieces, which include just about every stream and river valley, stream and river direction change, stream and river tributary, eroded upland surface, water gap, wind gap, abandoned valley, erosional escarpment, and drainage divide. To illustrate how each such feature must be considered as a relevant puzzle piece this essay will use the topographic map section seen in figure 1 which shows a section of northwest Philadelphia and some adjacent suburban areas. The figure 1 puzzle can be solved (however by expanding the map area the inclusion of many additional puzzle pieces will complicate the puzzle and the solution).

The brown lines on the map are contour lines connecting points of equal elevation and which on the figure 1 map are drawn at 10-foot intervals. Some contour lines are labelled to show their elevation. Closely spaced contour lines show steep slopes. Less closely spaced contour lines show gentler slopes and areas with no contour lines are level or nearly so. Perhaps the most obvious drainage feature in figure 1 is the Schuylkill River which flows across the figure 1 map in a southeast direction in a relatively narrow steep-sided valley before turning to flow in a southwest direction. Contour lines can be used to determine that the southeast-oriented Schuylkill River valley has been eroded into an upland surface which today has an elevation approximately 250 to 300 feet above the Schuylkill River valley floor.

The Schuylkill River valley and the upland surface into which the valley has been eroded are just two of the most obvious puzzle pieces seen in figure 1. Combined with geology’s principle of cross cutting relationships these two puzzle pieces describe a simple sequence of geologic events. The principle of cross cutting relationships states that a geologic feature cutting across another geologic feature is younger than the geologic feature which it cuts. Using this principle, it is not difficult to determine from figure 1 that the Schuylkill River valley has been eroded into what must have been preexisting upland surface. Other puzzle pieces are needed to determine how the Schuylkill River valley was able to turn from a southeast direction to a southwest direction and to erode a 250-300-foot-deep gorge into what was a preexisting upland surface.

Another obvious figure 1 puzzle piece is the narrow south-southeast oriented Wissahickon Creek valley which turns in a southwest direction before reaching the southeast-oriented Schuylkill River valley. The Wissahickon Creek valley is almost as deep as the Schuylkill River valley and must have also been eroded into a preexisting upland surface. But, adding the Wissahickon Creek valley as a puzzle piece introduces additional questions, such as why does the Wissahickon Creek valley change from a south-southeast direction to a southwest direction before joining the Schuylkill River valley and was the Schuylkill River valley or the Wissahickon Creek valley eroded first or were the two valleys eroded at approximately the same time?

Figure 1. Modified topographic map from the United States Geological Survey National Map website showing the Schuylkill River and Wissahickon Creek gorges in northwest Philadelphia and the adjacent suburban areas. The contour interval is 10 feet.

To answer these and other questions the three rather obvious puzzle pieces must be broken down into smaller puzzle pieces. The Schuylkill River valley puzzle piece can be broken down into a narrow southeast-oriented valley segment puzzle piece and a somewhat less deep southwest-oriented valley segment puzzle piece with the narrow and deeper southeast-oriented valley segment puzzle piece being joined by southwest-oriented tributaries from the northeast (which are more puzzle pieces) and northeast-oriented tributaries from the southwest (which are still more puzzle pieces). Likewise, the Wissahickon Creek valley can be broken down into a narrow south-southeast oriented valley puzzle piece and a narrow southwest-oriented valley puzzle piece with the narrow south-southeast oriented valley puzzle piece being joined by southwest-oriented tributaries from the northeast and northeast-oriented tributaries from the southwest.

Another important, but less obvious puzzle piece is the Wissahickon Creek-Schuylkill River drainage divide. Ridge Avenue is the red road passing through Roxborough which follows that drainage divide from the top of figure 1 in a south and southeast direction before leaving the divide and turning in a south direction to descend into the Schuylkill River valley. Unfortunately, but as is common in the Philadelphia metro region, urban development has obscured some of the drainage divide evidence. However, by looking closely it is possible to see shallow valleys (defined here as divide crossings) connecting southwest-oriented Schuylkill River tributary valleys with northeast-oriented Wissahickon Creek tributary valleys. Minor dips along Ridge Avenue such as seen in figure 2 identify some divide crossing locations. The divide crossings on this map section are subtle features but identify places where water once flowed across what is now the drainage divide between Wissahickon Creek the Schuylkill River. 

Figure 2. Google maps image of a Roxborough gentle dip in Ridge Avenue where a shallow valley or divide crossing crosses the Wissahickon Creek-Schuylkill River drainage divide.

Even where the divide crossing evidence is now obscured the conclusion that water once flowed across the Roxborough area Wissahickon Creek-Schuylkill River drainage divide can be reached by using a completely different line of evidence. Bedrock underlying the Roxborough area which is exposed along both the Wissahickon Creek and Schuylkill River valley walls can be demonstrated by a variety of different dating methods to be much older than surrounding region bedrock. Exposure of the older bedrock on the surface means water has removed whatever thicknesses of younger rock that once covered the Roxborough area drainage divide area. Whether all of the younger bedrock was removed at one time or at a number of different times does not affect the puzzle solution, which simply requires water to have once flowed across what is now the Roxborough Wissahickon Creek-Schuylkill River drainage divide area.

Knowing that water eroded the Roxborough upland surface means the now partially obscured divide crossings and Wissahickon Creek and Schuylkill River tributary orientations in northeast and southwest directions can be used to determine that the final Roxborough upland surface erosion event involved large amounts of water flowing in either a northeast or a southwest direction. Wissahickon Creek and the Schuylkill River now flow in south directions so it is probably safe to assume that southwest-oriented water was responsible for the final Roxborough upland surface erosion event. Southwest-oriented water could not have flowed across the Roxborough upland surface without also flowing across what is now the deep Wissahickon Creek gorge, which means the Wissahickon Creek gorge did not exist at that time which means the Wissahickon Creek valley erosion occurred after Schuylkill River valley erosion.

The puzzle solution has determined so far that the southeast-oriented Schuylkill River valley segment was eroded before the final Roxborough upland surface erosion event which was before erosion of the south-southeast oriented Wissahickon Creek gorge. Logic also suggests the southwest-oriented Schuylkill River valley segment existed before the southeast-oriented Schuylkill valley segment was eroded and the southwest-oriented Wissahickon Creek valley segment existed before the south-southeast oriented Wissahickon valley segment was eroded. At this point it is necessary to determine if the above-described events occurred independently or if they all were related to the southwest-oriented water event?

Southwest-oriented water during that final upland surface erosion event must have flowed into the southeast-oriented Schuylkill River valley which explains why Schuylkill River tributaries, including the southwest-oriented Wissahickon Creek segment, eroded southwest-oriented valleys. The Schuylkill River turn in a southwest direction and the northwest-oriented valleys of tributaries draining to the southeast-oriented Schuylkill River valley segment suggest the southwest-oriented water once also flowed across what is now the southeast-oriented Schuylkill River valley seen in figure 1, just as the divide crossings seen along the Wissahickon Creek-Schuylkill River drainage divide and the northwest- and southeast-oriented tributaries now draining into the south-southeast oriented Wissahickon Creek gorge suggest that southwest oriented water once flowed across the deep Wissahickon Creek valley.

Occam’s Razor is a rule in science which states that unless there is good reason to believe otherwise the simplest explanation which explains all relevant evidence should be preferred. It is certainly possible to explain erosion of the southwest- and southeast-oriented Schuylkill River valley segments and their tributary valleys, the southwest and south-southeast oriented Wissahickon Creek valley segments and their tributary valleys, the Roxborough upland surface and the divide crossings along the Wissahickon Creek-Schuylkill River drainage divide as separate events. It is also possible to use a much simpler explanation where the same southwest-oriented water event was responsible for eroding all of the mentioned landscape features.

Applying Occam’s Razor suggests the puzzle can now be solved by determining how large volumes of southwest-oriented water (from somewhere which cannot be determined from the figure 1 evidence) eroded the figure 1 landform features. It is easy to see how southwest-oriented water eroded southwest-oriented-oriented Schuylkill River and Wissahickon Creek valley segments, southwest-oriented tributary valleys now draining into the southeast-oriented Schuylkill River segment and the south-southeast oriented Wissahickon valley segment, and the divide crossings along the Wissahickon Creek-Schuylkill River drainage divide. However, how southwest-oriented water eroded the southeast-oriented Schuylkill River valley segment, the south-southeast oriented Wissahickon Creek valley segment, and the northeast-oriented Schuylkill River and Wissahickon Creek tributaries is not as obvious.

The southeast-oriented Schuylkill River valley and south-southeast oriented Wissahickon Creek valley segments are both 250-300-foot-deep and steep-sided narrow gorges which have been sliced into the adjacent upland surface. Tributary valleys while numerous are generally short, have steep gradients, and are usually oriented in either northeast or southwest directions. The steep gorge walls and steep tributary gradients suggest both gorges eroded headward into the upland surface in much the same way that Niagara Falls seen in figure 3 is eroding the Niagara Gorge headward along the Niagara River route. Note in figure 3 how the deep Niagara River Gorge has eroded headward in south-southwest direction to capture a west-flowing river just as the steep-sided southeast-oriented Schuylkill River valley and south-southeast oriented Wissahickon Creek valley segments must have eroded headward to capture southwest-oriented water.

Figure 3. Imagery from the United States Geological Survey National Map website showing Niagara Falls and the downstream narrow Niagara River Gorge. Note the wider upstream river and how the gorge has eroded headward at an angle to the upstream river direction.

Tremendous amounts of water probably flowing for 10,000 or more years eroded the Niagara River Gorge headward to where Niagara Falls is today. While Niagara region bedrock is different from the bedrock underlying the figure 1 Philadelphia map area the steep-sided Schuylkill River and Wissahickon Creek valleys seen in figure 1 suggest immense volumes of southwest-oriented water eroded those valleys headward in manner very similar to what is happening today at Niagara Falls, although whatever water falls that existed have migrated upstream and are no longer visible. How fast the Schuylkill River and Wissahickon valleys were eroded would have been determined by the volumes of southwest-oriented water flowing across the figure 1 map area which must have been comparable to or greater than the volumes of water that have been eroding the Niagara River Gorge headward.

Headward erosion of a deep southeast or south-southeast oriented valley across huge volumes of southwest-oriented water flowing in shallow diverging and converging channels where water can move easily from one channel to another would cause significant reversals of flow in newly beheaded channels. The reversed flow would move in a northeast direction into the newly eroded and much deeper southeast-oriented valley and if enough water was involved would erode a northeast-oriented tributary valley. A deep northeast-oriented tributary valley such as the Mill Creek valley in figure 1 will only be eroded if immense volumes of southwest-oriented water is still flowing to the northwest of the actively eroding valley head and is able to spill from the yet to be beheaded and reversed channels into a beheaded and reversed flood flow channel.

The figure 1 puzzle solution involves immense southwest-oriented volumes of water that flowed across what is now an upland surface while deep Schuylkill River valley and subsequently deep Wissahickon Creek valley headward erosion captured the southwest-oriented flow and diverted the water into what is now the Delaware River valley at Philadelphia. Volumes of water needed to erode the Schuylkill River and Wissahickon Creek valleys in just the figure 1 map area must have been comparable to or greater than the volumes of water needed to erode the Niagara River Gorge seen in figure 3. Where the water came from cannot be determined from the figure 1 evidence nor does the figure 1 evidence tell when the erosion occurred.  Whether this puzzle solution is consistent with the solutions to other Philadelphia area drainage system and erosional landform puzzles will be determined in subsequent blogs.