Figure 1: Bowman Hill at Washington Crossing Historic Park. The Delaware River flows in a southeast direction in the water gap it eroded between Bowman Hill and Belle Mountain. United States Geological Survey topographic map digitally presented using National Geographic TOPO software.
Bowman Hill is a high point (over 400 feet while the adjacent Delaware River is flowing at an elevation of less than 50 feet) composed of erosion resistant bedrock that stands above surrounding regions that are underlain by less erosion resistant materials. The Pennsylvania Geological Survey web applications map shows bedrock surrounding Bowman Hill to be composed of mudstone, shale, and siltstone of Triassic age while bedrock found at Bowman Hill is a fine-grained igneous rock known as diabase. The diabase was intruded into the surrounding bedrock during the Mesozoic Era. At that time rocks exposed in the Bowman Hill area today were not found at the surface as they are today and those rocks have been since tilted and uplifted and then exposed as the region was deeply eroded to achieve the landforms seen in figure 1.
Note Belle Mountain (elevation greater than 300 feet) in the figure 1 northeast corner and on the northeast side of the southeast oriented Delaware River. Belle Mountain is a northeast extension of the Bowman Hill intrusion and the Delaware River valley between Bowman Hill and Belle Mountain is a 250-350 foot deep water gap that was eroded at the time the southeast oriented Delaware River valley eroded headward across the region. Headward erosion of the Delaware River valley occurred at a time when the entire region was at least as high or higher than the present day top of Bowman Hill. At that time massive and prolonged southwest oriented floods moved across the entire region in anastomosing complexes of shallow diverging and converging channels permitting water to move freely between the flood flow channels. The southwest oriented floods were responsible for lowering the landscape surrounding Bowman Hill at the same time as the deep southeast oriented Delaware River valley was eroding headward into the region.
Southwest oriented flood flow channels were located both south and north of Bowman Hill. Delaware River valley headward erosion reached the southern flood flow channels first and beheaded the channel on the Bowman Hill southeast flank. Water on the northeast end of that beheaded flood flow channel reversed flow direction to flow in a northeast direction to reach the much deeper Delaware River valley. Headward erosion of the deep Delaware River valley next carved the water gap between Bowman Hill and Belle Mountain and beheaded and reversed flood flow channels north of Bowman Hill. Reversed flow on the northern channels captured significant flood flow from north of the actively eroding Delaware River valley head and eroded the northeast-oriented and deeper Pidcock Creek valley seen just north of Bowman Hill (see Pidcock Creek drainage basin entry on this website for a detailed discussion). The deeper Pidcock Creek valley also captured reversed flow that had been moving to the reversed flood flow channel south of Bowman Hill and that captured water moved in a north direction just west of Bowman Hill (note north oriented Pidcock Creek tributary west of Bowman Hill).