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The geologic history of the surficial units of the Fairmount and Rehoboth Beach quadrangles is that of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology reflects this complex history both onshore, in Rehoboth Bay, and offshore. Erosion during the late Pleistocene sea-level low stand and ongoing deposition offshore and in Rehoboth Bay during the Holocene rise in sea level represent the last of several cycles of erosion and deposition.
The Delaware Geological Survey has a continuing program to map the geology of the entire state at the detailed scale of 1:24,000. The STATEMAP component of the National Cooperative Geologic Mapping Program has contributed significantly to our surficial geologic mapping program. This work has resulted in not only new geologic mapping, but also the digital compilation of previous mapping. Products of this program include file formats that can be downloaded and printed from the web as geologic map products and imported into GIS software as georeferenced layers.
The Nanticoke River Group consists of the Turtle Branch and Kent Island Formations. The Nanticoke River Group consists of heterogeneous units of interbedded fine to coarse sand, clayey silt, sandy silt, and silty clay. Where the units are muddy, downstream of Seaford, the sequence consists of a lower fluvial to estuarine swamp to tidal stream deposits (coarse sand to gravelly sand with scattered organic-rich muddy beds) overlain by estuarine clayey silts and silty clays that contain rare to common Crassostrea (oyster) bioherms. The silts and clays are overlain by sands with clay laminae, to fine to coarse well-sorted, clean sand that are estuarne beach and eolian in origin. Upstream, the mud beds are rarer and restricted to the west side of streams and consist of organic rich clayey silt. Most of the stratigraphic section is dominated by clean, well-sorted sands.
The Assawoman Bay Group consists of the well-sorted sands, silts, and clays of the Omar, Ironshire, and Sinepuxent Formations found adjacent to and inland of the Atlantic Coast of Delaware and Maryland. These deposits in Delaware and Maryland were named from oldest to youngest: the Omar Formation (Jordan, 1962, 1964), the Ironshire Formation (Owens and Denny, 1979a), and the Sinepuxent Formation (Owens and Denny, 1979a).
Owens and Denny (1979) named the Kent Island Formation for deposits bordering the Chesapeake Bay found underneath lowlands that ranged in elevation from 0 to 25 feet in elevation but most of the land surface area is less than 10 feet in elevation. These lowlands are bordered by a scarp with at toe at approximately 25 feet. In its type area, the Kent Island Formation was described as consisting of thick beds of loose, light colored, cross-stratified sand overlying dark-colored massive to thinly laminated clay-silt. Pebbles as much as 10 cm (4 in.) in diameter occur in thin beds with the sand or as scattered clasts in both the sand and clay-silt. Locally, large tree stumps in growth position are encased in the clay-silt. Maximum thickness of the Kent Island was about 12 m (40 feet).
The Omar Formation was originally described (Jordan, 1962) as consisting of interbedded, gray to dark gray, quartz sands and silts with bedding ranging from a few inches to more than 10 feet thick. Thin laminae of clay are found within the fine, well-sorted sands. Silt mixed with sand generally contains some plant matter and where dark in color could be considered organic. Sands contain wood fragments, some of which are lignitic.
The Ironshire Formation was described by Owens and Denny (1979) as consisting of a lower loose, pale-yellow to white, well-sorted, medium sand characterized by long, low-angle inclined beds with laminae of black minerals. The upper portion of the units was described as consisting of light-colored, trough cross-stratified, well-sorted sand with pebbles and a few Callianassa borings. They described the Ironshire Formation near Rehoboth in a stratigraphic section which is now considered to be a part of the Lynch Heights Formation.