Time and space equivalency between the well-studied Neogene beds of Virginia and Maryland and those to the northeast in New Jersey are only generally understood. Diatom and foraminifer assemblages from a recently cored hole through the Cohansey Sand and underlying Kirkwood Formation indicate that Kirkwood is the same age as the Calvert Formation of Maryland (early to early middle Miocene). The Cohansey yielded no mega-invertebrates; however, pollen studies showed that the Kirkwood and overlying Cohansey have similar microfloras. The Cohansey is probably also middle Miocene in age and equivalent to the Choptank Formation of Maryland.

In New Jersey, the Cohansey is as high as 300 ft above sea level, with no Miocene or Pliocene deposits above it. In Virginia and Maryland, the Choptank is generally below 200 ft and is overlain by higher Miocene and Pliocene marine deposits in a series of small basins. The Cohansey and Kirkwood represent deposition in an older Miocene basin that was a highland when younger Miocene and Pliocene sediments were deposited in basins to the south.

The altitudes and areal distribution of the Miocene and Pliocene formations from New Jersey to Virginia suggest that tectonic movements along the continental margin have controlled sediment supply and sites of deposition. Eustatic changes in sea level only slightly modified this pattern.

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