Stop 4: Chickies Rock
Chickies Rock is an east-facing cliff cut into Chickies Ridge, held up by the Chickies Formation, a Cambrian sandstone that rests directly on mid-Proterozoic gneisses. This unit is stratigraphically equivalent to the Potsdam Sandstone in New York State, and the Cheshire Quartzite in western New England. The rock has been deformed, with open folds, cleavage development, and fossils exposed. Chickies Rock is right next to the Susquehanna River, a very busy railroad track, and it’s a highly popular rock climbing cliff.
View of an open anticlinal fold in the layered sandstone. Though subtle in this image, fanning of the axial planar cleavage is visible.
Open, anticline in the layered sandstone. The axial planar cleavage is quite visible here.
Another, larger anticline in the layered sandstone, with rock climber up above. Axial planar cleavage is visible.
A large fraction of the folding here was accommodated by flexural slip, where the competent sandstone layers sliped past one another along shaley interbeds. Here, the nearly vertical axial planar cleavage in the sandstone layers bends sharply into the shaley slip surfaces above and below the sandstone. In the shaley layers, cleavage dips ~30° to the right (south).
Another view of nearly vertical axial planar cleavage in sandstone beds, which bends sharply into the plane of shaley interbeds which are the flexural slip surfaces.
These relatively broad wavy features are ripple marks on a sandstone bedding surface. Their spacing is ~5 cm.
Somewhat similar to ripples are these cleavage-bedding intersection lineations, which were originally misidentified as ripple marks. The spacing here is ~1 cm.
These rocks are absolutely full of the trace fossils shown here, which are burrows of the annelid worm Skolithos linearis. The burrows are 4-5 mm in diameter and perpendicular to the sandstone beds. When I was an undergraduate in 1976, I worked for the USGS in Reston, VA. I helped mapping landslides in Cretaceous coastal plain sediments in Annandale, just south of Washington D.C. The highlands of the coastal plain sediments were held up by a layer of “upland gravel”, which was a coarse, poorly-cemented cobble conglomerate. Many of the quartz sandstone cobbles had trace fossils exactly like these. The burrows in the cobbles were flattened, typically 5 mm by 3 mm in cross section, clear evidence of deformation. I was amazed to see the probable source unit for those cobbles, a few of which I still have in my collection.
