40 37' 32 N
75 27' 50 W

As the Lehigh River flows south from the Lehigh Water Gap, it begins to enter the Lehigh Valley. At this point, the river flows across the valley and into Allentown where it quickly turns eastward and follows the line of the Lehigh Mountains (South Mountain) to the Delaware River.

When the river reaches Allentown it flows across Precambrian gneiss geology of the Lehigh Mountains. Unable to erode the hard rock, the Lehigh is forced to turn left. Because the direction of the river’s flow is determined by structural geology, this section of the river is referred to as “subsequent.” Sections where the river is able to flow southward along its original slope, uninhibited by geologic structures are called “consequent.”

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