Neshaminy Creek Water Gap and Elbows of Capture near Langhorne

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The Neshaminy Creek drainage route north and west of Langhorne exhibits several intriguing elbows of capture. Figure 1 below shows Neshaminy Creek flowing in a southeast direction to Bridgetown (near map northeast corner) where it then makes a U-turn to flow in a west-southwest direction to Playwick Park (near map center). East-northeast oriented Mill Creek joins Neshaminy Creek at Playwick Park as a barbed tributary and Neshaminy Creek then turns abruptly to flow in a south, southeast, and southwest direction to the map south center edge.

NeshaminyLanghornelbows

Figure 1: Neshaminy Creek elbows of capture and water gap near Langhorne. See text for description. United States Geological Survey map digitally presented using National Geographic TOPO software. 

The west-southwest oriented Neshaminy Creek segment and east-northeast oriented Mill Creek are flowing in a linear valley that is probably related to a linear geologic feature (the Pennsylvania Geological Survey web application map shows the valley located along the boundary between two different metamorphic rock units, although both units should be resistant to erosion and are resistant to erosion both north and south of the boundary). The linear valley continues in an east-northeast direction to the Neshaminy Creek-Delaware River drainage divide located east of figure 1. That drainage divide elevation is between 160 and 170 feet. Yet instead of flowing to join the Delaware River, Neshaminy Creek after flowing in a west-southwest direction turns to flow in a narrow south oriented valley or water gap bounded by uplands rising to more than 200 feet on either side.

A question worth answering is why does Neshaminy Creek flow in a west-southwest and south direction across a 200-foot high upland rather than in an east-northeast direction along the lower elevation linear valley to reach the Delaware River? A logical explanation is the Delaware River valley east of figure 1 and the linear valley did not exist at the time the deep Neshaminy Creek valley eroded headward into the figure 1 map region. At that time the entire region was as high or higher than the highest elevations seen in the figure 1 map area today (more than 200 feet in terms of today’s elevations, although at that time sea level may have been significantly lower) and the present day valleys did not exist. Also at that that time massive and prolonged southwest oriented floods were moving across the region. Floodwaters came from east and north of the present day south oriented Delaware River valley, which today is found east of figure 1.

When the deep Neshaminy Creek valley eroded headward in the high-level surface that once existed throughout the figure 1 map region the southwest oriented floodwaters were eroding the deep southwest-oriented Delaware River valley found south of figure 1. The deep south-oriented Neshaminy Creek valley eroded headward from that actively eroding southwest-oriented Delaware River valley to capture southwest oriented flood flow moving in the region north of the actively eroding Delaware River valley. In the case of the Playwick Park elbow of capture the actively eroding south-oriented Neshaminy Creek valley captured a major west-southwest oriented flood flow channel (perhaps where flood flow was concentrated along a zone of easily eroded bedrock) and then eroded headward in an east-northeast direction along that west-southwest oriented flood flow channel.

By capturing the west-southwest oriented flood flow channel the deep south-oriented Neshaminy Creek valley diverted the floodwaters in a south direction to the newly eroded southwest oriented Delaware River valley and at the same time beheaded the west-southwest oriented flood flow channel. East-northeast Mill Creek was formed when flood flow on the east-northeast end of the beheaded flood flow channel reversed flow direction to flow to the much deeper south-oriented Neshaminy Creek valley. But, large volumes of southwest oriented floodwaters were still flowing across the region north of the figure 1 map area. South and southeast oriented valleys eroded headward from the beheaded and reversed linear flood flow-channel to capture the flood flow moving further to north.

Pine Run, seen in figure 1, illustrates how headward erosion of a south oriented valley from the Playwick Park area captured a west-southwest oriented flood flow channel and also reversed flow on the east-northeast end of the beheaded flow channel. Note how the south oriented Pine Run headwaters valley eroded in a north direction to capture still more flood flow, which helped erode the east-northeast Pine Run valley segment seen today. The same situation occurred with reversed flow on the present day Mill Creek alignment. West of figure 1 the south oriented Ironworks Creek valley and the southeast oriented Mill Creek headwaters valley eroded headward to capture southwest oriented flood flow channels further to the north and to divert that water to the newly formed linear flood flow channel and to erode the east-northeast oriented Mill Creek valley.

The deep south and southeast oriented Neshaminy Creek valley from the Bridgetown elbow of capture also captured flood flow north of the linear flood flow channel and first beheaded flow to the newly eroded Pine Run valley and subsequently beheaded the flood flow channels supplying floodwaters to the south oriented Ironworks Creek valley (which had already beheaded flood flow channels to the southeast oriented Mill Creek headwaters valley). Southwest oriented flood flow across the region was further beheaded when the deep Delaware River valley turned from eroding headward in a northeast direction along a southwest oriented flood flow channel to eroding headward in a north and northwest direction across the southwest oriented flood flow channels. Headward erosion of the southeast and south oriented Delaware River valley east of figure 1 ended all southwest oriented flood flow to the newly eroded Neshaminy Creek valley and created the Neshaminy Creek drainage basin as we see it today.

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