[The Hamburg Stromatolites and the Great Unconformity, New Jersey, May 5, 2026]

[May 5, 2026; Hamburg, New Jersey] I attended the Department of Earth and Environmental Sciences Retreat, held at the Mineral Hotel and Spa in Vernon Township, New Jersey. During the afternoon break, my colleague Sidney Hemming led us on a short fieldtrip to view two nearby geological attractions, the Hamburg Stromatolites and the Great Unconformity. We carpooled; Roger Buck kindly drove me and several others. Stromatolites are algal mounds that grow in shallow marine environments, and have a shape that is similar to a coral head. These stroms, located at 30 Bluffs Court, Hamburg, New Jersey, occur in the Allentown Dolomite (rock) and are about 500 million years old. They are exposed on the steep west side of a glacially-polished whaleback ridge, and are easy to view, as they occur just a few feet above ground level. Perhaps a hundred stroms are spectacularly preserved, and appear as light-colored circular features with concentric growth rings, like those of a tree. They vary in size from several inches to several feet. Sidney, Roger Buck, Fola Kolewale, Bärbel Hönisch, Terry Plank, me and about a dozen others spent about a half hour prowling all over the whaleback, inspecting the stroms, the dolomite in which they are imbedded, and the striae that glaciers left on the surface of the whaleback. We then drove to nearby Emerald Drive, where a road exposes 1,100 million-year-old Proterozoic gneisses overlain with shallowly dipping layers of 500 million-year-old Paleozoic sandstone and limestone. This locale had been billed as an example of the Great Unconformity – where erosion had destroyed 600 million years of the geologic record. However, when I inspected the boundary between the gneiss and the sediments exposed on the west side of the road, the presence of a thin layer of gouge indicated to me that the boundary isn’t an unconformity at all, but a geological fault. The exposure on the east side of the road is poorer, but the rocks there show signs of yellow-colored hydrothermal alteration, which is also typical of a fault but not of an unconformity. Fola, who is a structural geologist, agreed with my interpretation and identified the feature as a normal fault. Later, however, my Lamont colleague Paul Olsen said that such bedding plan faults are not uncommon and that its presence should not detract from calling the feature and unconformity. I also found a second fault with a more vertical fault plane and prominent slickensides, located about a hundred yards to the north on the east side of the road. Fola identified it as a left-lateral strike slip fault. Pretty Red Columbines were blooming by some of the rock outcrops. 1:00.