COLLUVIUM AND PLEISTOCENE HILLSLOPE EROSION IN THE NEW JERSEY HIGHLANDS

The New Jersey Highlands are an upland with 90 to 300 m of local relief underlain mostly by gneiss. Colluvium derived from weathered gneiss is the dominant surficial deposit in the Highlands south of the late Wisconsinan (LGM) glacial limit. It is mostly matrix-supported clayey-silty sand diamicton with angular to subangular clasts of gneiss. Tabular clasts have strong slope-parallel fabric. This diamicton in places overlies thinly layered clayey sand that is derived from saprolite and grus. This stratigraphy is from colluvial inversion of saprolite over fractured rock on the feeder slope. On toeslopes the diamicton may also overlie, or interbed with, thinly layered clayey silt and sand deposited from groundwater seepage and slopewash. These deposits form footslope aprons as much as 25 m thick that typically consist of upper, lightly weathered colluvium as much as 10 m thick over lower, weathered colluvium of similar or greater thickness. The upper colluvium is of LGM age based on peat dated to 40.7 cal ka under the colluvium, basal organic silty clay dated to 18.5 cal ka in a bog dammed by the colluvium, and interbedding of the colluvium with LGM glaciofluvial and glaciolacustrine deposits. The lower colluvium has clast weathering and soil development like, or more intense than, that in Illinoian glacial deposits, and is interbedded with, and underlies, Illinoian glaciolacustrine deposits. In a few places the weathered colluvium is on grade with, and contains erratic clasts from, till of early Pleistocene age. The stratigraphy and sedimentary properties of the colluvium indicate that it is deposited episodically during periods of cold climate by solifluction, creep, and slopewash. Closed forest under temperate climate anchors slopes and allows colluvium to weather in situ. Gullying and bank erosion partially erode colluvium under temperate climate but the thickness and pervasive stacked stratigraphy indicates that it mostly accumulates and remains on footslopes. Episodic headslope erosion through the Pleistocene caused retreat of headslopes and pedimentation of the rock surface beneath footslopes. The volume of footslope colluvium at 16 sites indicates vertical erosion depths between 4 and 30 m and horizontal retreat distances between 20 and 100 m on headslopes in the Pleistocene.