An interesting New Jersey landform providing significant information related to Philadelphia area landforms is the Millstone River water gap located near Princeton. The Millstone River flows in a northwest direction, as seen at location 1 in figure 1, to enter Carnegie Lake at Princeton and then to flow in a north direction to location 2. Location 2 is where the Millstone River flows through the water gap separating Rocky Hill and Little Rocky Hill. Elevations on the Rocky Hill-Little Rocky Hill ridge exceed 300 feet on either side of the water gap, yet the low gradient Millstone River crosses the 40-foot contour line just north of the gap. North figure 1 the Millstone River flows to the southeast and east oriented Raritan River, which flows to Raritan Bay (south of Staten Island). While not seen in figure 1 elevations to the north and east of Little Rocky Hill decrease to about 100 feet and provide an extensive low elevation plain that offers the Millstone River significantly shorter routes to the Raritan Bay that completely avoid having to cross the 300-foot high Rocky Hill and Little Rock Hill basalt ridge.
Figure 1: Millstone River water gap area near Princeton, New Jersey. See text for discussion. United States Geological Survey map digitally presented using National Geographic TOPO software.
Further confusing the Millstone River route is the route of Stony Brook, which flows in a south direction through location 3 (west of Princeton) to location 4 and south of figure 1 abruptly turns to flow in a northeast direction to Carnegie Lake and to join the north oriented Millstone River. The solid blue line extending from location 4 to Carnegie Lake and then to location 2 is the Delaware and Raritan Canal, which interestingly was kept full with water flowing in a north direction from the Delaware River. The Stony Brook abrupt direction change and the ability of Delaware River water to keep the Delaware and Raritan Canal full suggest the low gradient Millstone River originated as a south and/or southwest oriented river and subsequently reversed its flow direction.
If correctly interpreted erosion of the Millstone River water gap near Princeton by south and southwest oriented flow is supportive of the interpretation that massive and prolonged southwest oriented floods eroded Philadelphia area landforms. Erosion of the Millstone River water gap would have begun at a time when floodwaters were flowing at an elevation of at least 300 feet (in terms of present day sea level), which means areas north and east of Little Rocky Hill (and north and east of figure 1) were also at least as high. In other words the Millstone River water gap provides evidence of south and southwest oriented flow across central New Jersey at an elevation of at least 300 feet, which would have been adequate to cross at least some drainage divides west of the present day Delaware River valley. The southwest oriented floodwaters would have eventually eroded the lower Delaware River valley and also lowered the landscape surrounding Rocky Hill and Little Rocky Hill by more than 200 feet.
Several previous researchers have commented on evidence for the reversal of flow on the Millstone River route and discussed additional evidence that cannot be seen in figure 1. William Morris Davis in his paper titled “The Rivers of Northern New Jersey” made the following comment: “It [the Millstone River] appears still to lie for the most part in the channel that it occupied before the elevation and tilting of the base-leveled central plain, but the tilting of the plain seems to have reversed its direction of flow. It rises near the center of the state and flows northwestward till it joins the Raritan near Somerville, and on the way it crosses from the thrown or depressed to the heaved or elevated side of the fall line, and passes through a deep gap in the trap ridge of Rocky hill, back of Princeton. I believe there is no other Atlantic river which runs against the fall line in this way, and it is certainly at first sight remarkable that a stream of moderate size like the Millstone should have held its own against a displacement that sufficed to deflect great rivers like the Delaware and the Susquehanna from their courses.”
Scott Stanford of the New Jersey Geological Survey in a 1997 paper written for the Northeastern Friends of the Pleistocene 60th Annual Reunion discusses a river system that “incised broad valleys and lowlands as much as 200 feet below the Beacon Hill level and deposited the Pensauken Formation”. He goes on to say “Paleocurrent measurements in the Pensauken show a consistent and strong southwest paleoflow…. The base-of-Pensauken contours, when projected north of the late Wisconsinan limit, intersect the floors of paired wind gaps through the double Watchung basalt ridges at Milburn and Paterson, and the wind gap at Sparkill, New York, through the Palisades diabase ridge…. The main axis of the Pensauken valley projects across a broad low in the Palisades between Jersey City and uplands on Staten Island…. The accordance of the gaps with the Pensauken suggests that they mark the route of the Hudson, perhaps superposed from the Beacon Hill fluvial plain, as a tributary to the Pensauken, with the trunk Pensauken river flowing southwesterly through the broad gap in the Palisades from the area of the Long Island Sound lowland. This trunk stream likely included drainage from much of southern New England. …Another site showing evidence of superposition is the Rocky Hill Gap, …where a south-flowing tributary to the Pensauken cut a gap through a diabase ridge. The floor of this gap has a lag of quartz pebbles and falls on the base-of-Pensauken contours. An inner gorge has been cut below the wind gap level by the north-flowing Millstone River, which established its course after the diversion of the Pensauken.”
 Davis, William Morris, 1909, Geographical Essays, Ginn and Company, New York, NY. p. 511.
 Sanford, Scott, D., 1997, Pliocene-Quaternary Geology of Northern New Jersey: An Overview, in Pliocene-Quaternary Geology of Northern New Jersey, Guidebook for the 60th Annual Reunion of the Northeastern Friends of the Pleistocene. pp.1-9 to 1-11.