23. Spruce Run State Park

The complexity of the geology of the Highlands along I-78 is partly due to the numerous great thrust faults in this region. The size and extent of these thrust faults can perhaps be best appreciated at Spruce Run Recreation Area, a picnic and camping area along the western shore of Spruce Run Reservoir. Spruce Run is located off of I-78 at exit 17. Follow 31 north about 2 miles and turn left on Van Syckels Road. The park entrance is 1.5 miles ahead on the left.

Just inside the park entrance are several large outcrops and glacial erratics consisting of Ordovician Leithville Dolomite. These exposures are the only rocks to look at in the park vicinity. Other exceptional exposures of the dolomite occur along the lake shoreline but are only accessible by boat. However, the hillsides on either side of this broad valley surrounding the reservoir consist of Precambrian gneiss and granite. Spruce Run Valley is part of the Jutland Klippe. The Precambrian rocks were thrust westward over the Paleozoic sedimentary rocks during the Taconic Orogeny. This great overthrust sheet was subsequently deformed during the Acadian and Alleghenian Orogenies. Since mid-Tertiary time, Spruce Run and other streams in the area have carved down through the overthrust sheet, exposing the underlying Paleozoic sedimentary rocks.