The Geology of Delaware

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Bryn Mawr Formation

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Geologic Time Period: 
Miocene
Coastal Plain - Primarily Subsurface Unit

The following description was published in GM13 Geologic Map of New Castle County, Delaware, Ramsey, K.W., 2005:

Reddish-brown to yellowish-brown silty quartz sand to sandy silt that interfingers with medium to coarse clayey sand with gravel. Sand fraction, where a sandy silt, is fine- to very fine-grained and angular to subangular. Iron-cemented zones are common. Gravel fraction is primarily quartz. Sands are quartzose with minor amounts of weathered feldspar. Opaque heavy minerals form up to 3 percent of the sand fraction. Unit ranges up to 70 ft thick but generally less than 30 ft thick and commonly less than 10 ft thick. Surface forms a distinctive terrace that has elevations between 350 ft and 425 ft, and it overlies saprolite of the Piedmont rocks. No macrofossils have been recovered. Fossil pollen from the York Pit in Cecil County, Maryland (Pazzaglia, 1993; unpublished DGS data) indicate a Miocene age. Owens (1999) considered the unit late Oligocene in Pennsylvania.

Reference(s): 

Owens, J.P., 1999, Cretaceous and Tertiary, in Shultz, C.H., editor, The Geology of Pennsylvania: Pennsylvania Geological Survey Special Publication No. 1, p. 219-223.

Pazzaglia, F.J., 1993, Stratigraphy, petrography, and correlation of late Cenozoic middle Atlantic Coastal Plain deposits: implications for late-state passive-margin geologic evolution: Geological Society of America Bulletin, v. 105, p. 1617-1634.