PLEISTOCENE EROSION OF SCHOOLEYS MOUNTAIN, NEW JERSEY

Paleotopography reconstructed from surficial deposits tracks landscape change over periods of 10⁴ to 10⁶ years and offers an independent measure of erosion that complements other methods. The distribution of pre-Illinoian, Illinoian (?), and late Wisconsinan glacial deposits on Schooleys Mountain, a gneiss plateau in northern New Jersey, and in adjacent carbonate valleys, marks landsurfaces at ~2 Ma, ~150 ka, and 20 ka, providing a means of quantifying Pleistocene erosion of the mountain. Overall relief increased by 50-80 m due to river incision and solution lowering in carbonate valleys. Upland valleys on gneiss were deepened 10-20 m and widened by groundwater seepage. This valley-upland mismatch in incision depth produced knickpoints on streams draining from the plateau. Almost all of the incision was accomplished in the early and middle Pleistocene, since Illinoian and late Wisconsinan deposits fill modern valley bottoms. Carbonate-valley denudation in the early and middle Pleistocene occurred at a maximum overall rate of 25 m/my; valleys on gneiss uplands lowered at a maximum overall rate of 8 m/my. Carbonate valleys deepen preferentially along the footslope of gneiss uplands where groundwater and runoff are most acidic. Upland-valley incision is inhibited by boulder lags formed by winnowing of gneiss regolith and colluvium. Post-Illinoian valley incision is limited to partial removal of Illinoian and Wisconsinan glaciofluvial gravel and periglacial colluvium, at a maximum overall rate of ~60 m/my. Periglacial hillslope erosion during the middle and late Pleistocene was greatest on long slopes steeper than 10^o, where footslope colluvial volumes indicate 5-20 m of headslope surface lowering.