ZONES IN MIOCENE DEPOSITS IN MARYLAND

In the Miocene report of the Maryland Geological Survey, G. B. Shattuck divided the Calvert, Choptank and St. Marys Formations into zones. The zones are not true paleontologic zones as that term is used today, but are mainly individual beds in the formations. A number of the zones cannot be certainly recognized in all of the outcrops. Shattuck was misled in places by the presence or absence or by the relative abundance of fossils, so that he based more than one of his zones on what was actually a variable horizon. For these reasons Shattuck's zone numbers are not used in this report. For those who attempt to utilize the zones, the following equivalences are noted:

1. Pycnodonte percrassa bed (Pl. 6, fig. 3), the basal bed of the Plum Point Marl Member of the Calvert Formation, is zone 4. The underlying Fairhaven Member comprises the first three zones.
2. The lower richly fossiliferous bed of the Calvert Formation is zone 10. Zones 5 to 9 cannot be certainly differentiated from each other.
3. The upper richly fossiliferous bed of the Calvert is zone 14. Zones 11 and 13 are essentially unfossiliferous clays. Zone 12 contains a considerable number of poorly preserved fossils that are usually too soft to be collected. It has, however, yielded more vertebrate fossils than any other zone. North of Parker Creek, zone 14 is located so high in the cliffs as to be practically inaccessible, but south of Parker Creek it has yielded a fauna that is composed primarily of Glossus fraterna Say variety marylandica Schoonover (Pl. 13, fig. 1, 2).
4. The lower abundantly fossiliferous bed in the Choptank Formation is zone 17 (Drumcliff Member). It is difficult, if not impossible, to distinguish zones 15 and 16 from each other, or inland from the cliffs, from the upper Calvert zones. Shattuck was probably misled by the varying amounts of clay and yellow sand in the transitional strata between the Calvert and Choptank Formations into assigning the strata at times to the top of the Calvert as zone 15 and at times to the base of the Choptank as zone 16.
5. The upper abundantly fossiliferous bed of the Choptank Formations is zone 19 (Boston Cliffs Member). The relatively unfossiliferous strata between the two abundantly fossiliferous beds in the Choptank Formation are zone 18.
6. The lower fossiliferous horizon in the St. Marys Formation which yields most of the fossils found at Little Cove Point is zone 22. Zone 20 is an unfossiliferous bed at the top of the Choptank Formation, and zone 21 is an unfossiliferous clay and sand sequence at the base of the St. Marys Formation. Zone 23 is a series of clays and fine sands that overlies zone 22 in the Calvert Cliffs section. Zones 20 through 23 are not distinct inland from the type section. In the latter, zone 23 is sparsely fossiliferous but does yield casts of bivalves and of Turritella plebia Say (Pl. 22, fig. 5, 6, 8). A thin band in this zone contains small concretions, some of which have yielded poorly preserved crab claws.
7. The fossiliferous strata on the shores of the southern part of St. Marys County, and especially along both sides of the St. Marys River, are zone 24.

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