Fort Washington Water Gaps (Wissahickon Creek and Sandy Run)

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Two adjacent water gaps can be seen just south of Fort Washington. The first, the Wissahickon Water Gap, is where Wissahickon Creek flows in a south direction between Fort Hill (east) and Militia Hill (west). The second, the Sandy Run Water Gap, is where Sandy Run turns from flowing in a west direction to flow in a north direction between Camp Hill (east) and Fort Hill (west) before turning to flow in a west and southwest direction to join Wissahickon Creek and then to flow through the Wissahickon Creek Fort Washington Water Gap. Transportation routes use both water gaps. Bethlehem Pike uses of the Wissahickon Creek Fort Washington Water Gap between Militia Hill and Fort Hill and the SEPTA Lansdale Regional Rail line, a Norfolk Southern freight rail line, Route 309, and the Pennsylvania Turnpike converge in the Sandy Run Water Gap.

Figure 1: Fort Washington area water gaps.

 

Figure 1: Fort Washington is the town just north of location 1. The Sandy Run Water Gap is found at location 1 and the Wissahickon Water Gap is found at location 4. Wissahickon Creek flows in a south direction along the map west edge. Location 2 identifies the elbow of capture where Sandy Run turns from flowing in a west direction to flow in a north direction through the Sandy Run Water Gap before turning to flow in a west direction to location 4. Location 3 shows a southwest oriented Sandy Run tributary. Fort Hill is at location 5 and Camp Hill is at location 6. Location 7 is the Edge Hill Water Gap cut into the northeast oriented Edge Hill Ridge, which marks the Whitemarsh Valley southeast margin. Camp Hill forms the Whitemarsh Valley north and northwest margin in the region east of Wissahickon Creek. United States Geological Survey map digitally presented using National Geographic software.

Camp Hill, Fort Hill, and Militia Hill are erosional residuals composed of what was probably a continuous band of erosion resistant rock located along the Whitemarsh Valley north margin. The Whitemarsh Valley to the south is underlain by less erosion resistant bedrock and is bounded on the south by erosion resistant bedrock. Elevations on the Militia Hill, Fort Hill, and Camp Hill discontinuous ridge today vary but are usually exceed 300 feet on Militia Hill and Camp Hill and are slightly less on Fort Hill. Wissahickon Creek crosses the 150-foot contour line as it flows between Militia Hill and Fort Hill and the Sandy Run elevation between Fort Hill and Camp Hill is only slightly higher. Depending on elevations used water gaps are between 130 and 200 feet deep.

The origin of each of the water gaps is an interesting question, although perhaps the most interesting question is why does water flow in a south direction between Militia Hill and Fort Hill, yet just one mile away water flows in a north direction between Fort Hill and Camp Hill? These questions cannot be answered in the context of present day topography, although the present day Sandy Run drainage basin provides some clues.

Sandy Run originates in Roslyn and flows in a west direction along the Camp Hill ridge south flank or the Whitemarsh Valley north side toward south oriented Wissahickon Creek, but instead of continuing in a west direction Sandy Run turns abruptly to flow in a north direction between Camp Hill and Fort Hill before turning to flow in a west direction to reach south oriented Wissahickon Creek. At its north elbow of capture (north of the Sandy Run Water Gap) southwest oriented tributaries join Sandy Run. Sandy Run drainage routes both south and north of the Camp Hill ridge are linked by through valleys with the south oriented Pennypack Creek valley to the east. The Pennsylvania Turnpike and the Norfolk Southern freight line make use of one such through valley found just north of the Camp Hill Ridge. Several through valleys in the Crestmont and Willow Grove area link the Whitemarsh Valley with the Pennypack Creek valley.

The through valleys provide evidence that water once flowed in a southwest and west-southwest direction from the present day Pennypack Creek drainage basin (probably across it) to the present day south oriented Wissahickon Creek valley. This southwest and west-southwest oriented flow was responsible for eroding less resistant bedrock on either side of the Camp Hill and Fort Hill (and Militia Hill) erosion resistant bedrock ridge and shaped the Camp Hill and Fort Hill (and Militia Hill) ridge we see today.

In other words, when the regional erosion began elevations on both sides of the Camp Hill and Fort Hill (and Militia Hill) ridge were at least as high as the highest ridge crests today. Prior to headward erosion of the deep Wissahickon Creek valley water flowed in a southwest and west-southwest direction to reach the south oriented Plymouth Creek valley (which drains to the Schuylkill River). But, the south oriented Wissahickon Creek valley could not have eroded headward across the Whitemarsh Valley and then further north unless southwest oriented flow was also moving to the Wissahickon Creek valley. In other words, while the region was being eroded the Wissahickon Creek and Plymouth Creek valleys were competing with each other to capture southwest and west-southwest oriented flow moving across the present day Sandy Run drainage basin.

Erosion events began when all elevations in the present day Fort Washington region were at least as high as the highest ridge crests and when headward erosion of the south oriented Wissahickon Creek valley was competing with headward erosion of southwest and south oriented Plymouth Creek valley to capture southwest and west-southwest oriented flood flow moving into the region. With those assumptions the Sandy Run Water Gap was probably initiated as a south oriented water gap by southwest and west-southwest oriented flow moving across the region now drained by the Sandy Run tributaries located north of the Camp Hill ridge. Not all of that water flowed through the eroding Sandy Run Water Gap and some water continued in a west-southwest direction along the north side of present day Fort Hill to the south oriented Wissahickon Creek Fort Washington Water Gap, which was being eroded at the same time.

Southwest and west-southwest oriented flow south of the Camp Hill, Fort Hill, and Militia Hill ridge lowered elevations south of the Camp Hill, Fort Hill, and Militia Hill ridge more rapidly than to the north of the ridge. Once the two water gaps were initiated flow from north of the Camp Hill, Fort Hill, and Militia Hill was concentrated in the two water gap locations, although water probably spilled across the ridge in other locations to cause what are today low points along the ridge crest and the lower elevation of Fort Hill when compared with the Camp Hill and Militia Hill ridge crest elevations.

Volumes of south oriented flow moving through the two adjacent water gaps was approximately equal until headward erosion of the south oriented Pennypack Creek valley beheaded all southwest oriented flood flow that had been moving to the newly eroded Sandy Run Water Gap and created the Sandy Run drainage basin as we see it today. The result was a significant reduction of south oriented flow moving through the Sandy Run Water Gap. Flow through the Wissahickon Creek Fort Washington Water Gap floor was being obtained from north of the Pennypack Creek valley head and continued to erode a deeper valley, which a caused a reversal of flow in the Sandy Run Water Gap. Reversed flow in the Sandy Run Water Gap also captured the then greatly reduced west oriented Sandy Run drainage route along the Camp Hill south margin to finish creating the Sandy Run drainage basin as we see it today.

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