A RELICT, LATE-WISCONSINAN, TERRACE-SLOPE LANDSLIDE FEATURE, FLAT BROOK VALLEY, SUSSEX COUNTY, NEW JERSEY, USA

Geomorphic mapping associated with a geology field camp conducted June, 2014 verified the presence of a relict landslide feature on the terrace slopes of Flat Brook, northwest New Jersey. The study area is within deglacial terrain of the Wisconsinan-age Kittatinny and Minisink Valley ice lobes. Flat Brook, a tributary of the Delaware River, traverses ice margin recessional moraines, and its valley is aggraded with bouldery alluvium. Flat Brook is currently entrenched ~15-20m into these sediments, possibly due to melt-water incision.

The landslide hollow is found along the incised ravine of Flat Brook. The origin of the landslide may be from periglacial thawing. Geomorphic assessment of the area included compass and tape transects, TOTAL station elevation surveys, shallow ground penetrating radar transects, and soil profile excavations. The amphitheater-shaped hollow is ~150m long and ~50m wide; the sides of the hollow terminate in small spur ridges, partially eroded by the stream. The current water table is relatively high, at the base of the hollow (<30cm) as well as along the slope and scarp (~70cm). Soils at the base of the hollow, now forming a low stream floodplain terrace, were typical of aggrading Holocene floodplain sediments. Soils along the scarp were thin but with some horizon development. Upper terrace surface soils were more complex and deeper. No landslide debris toe was present, presumably washed away by Flat Brook. There appears to be no recent large scale movement of the slide, apart from leaning trees. However, a recent and ongoing mudflow, ~5 meters wide, with a perennially high water table, emanates from the ridge spur to the west side of the landslide hollow.