Geologic units in Horry county, South Carolina

Socastee Formation (Pleistocene) at surface, covers 40 % of this area

Low coastal formation in Carolinas like Penholoway but younger and lower in altitude.

Bear Bluff Formation (Pliocene) at surface, covers 19 % of this area

One of the older coastal terrace sequences in the Carolinas. Equivalent to Windsor Fm.

Alluvial Valley Swamp (Quaternary) at surface, covers 18 % of this area

Unconformable on all underlying units, fluvial sand and gravel at base, grading upwards into fine sands and silts, local peat. May be overrun with recent sediments from forest cutting and agriculture.

Waccamaw Formation (Pleistocene) at surface, covers 11 % of this area

Another Carolina costalized terrace of early-middle Pleistocene age. Deeply weathered.

Penholoway Formation (Pleistocene) at surface, covers 8 % of this area

Similar to Cape May, broad lateral extent underlying terraces in the Carolinas; swamps and ridges on terrace surface were originally barrier islands and back bays. Superimposed on these landforms are swarms of Carolina bays.

Chenier plain and deltas of Suwannee & Chattahoochie Rivers (Holocene) at surface, covers 2 % of this area

Chenier plain and deltas of Suwannee & Chattahoochie Rivers

Yorktown Formation and Duplin Formation, Undivided (Tertiary) at surface, covers 0.3 % of this area

Yorktown Formation: fossiliferous clay with varying amounts of fine-grained sand, bluish gray, shell material commonly concentrated in lenses; mainly in area north of Neuse River. Duplin Formation: shelly, medium- to coarse-grained sand, sandy marl, and limestone, bluish gray; mainly in area south of Neuse River.

Swamp (Quaternary) at surface, covers 0.1 % of this area

Extensive cypress swamps occur in low-lying poorly-drained bay deposits. Limit of cypress is southern Delaware; from NJ south to Georgia. Atlantic white cedar is original species. Commonly extensive thick peat and buried wood.

Black Creek Formation (Cretaceous) at surface, covers < 0.1 % of this area

Clay, gray to black, lignitic; contains thin beds and laminae of fine-grained micaceous sand and thick lenses of cross-bedded sand. Glauconitic, fossiliferous clayey sand lenses in upper part.

Peedee Formation (Cretaceous) at surface, covers < 0.1 % of this area

Sand, clayey sand, and clay, greenish gray to olive black, massive, glauconitic, locally fossiliferous and calcareous. Patches of sandy molluscan-mold limestone in upper part.

Surficial Deposits, Undivided (Quaternary) at surface, covers < 0.1 % of this area

Sand, clay, gravel, and peat deposited in marine, fluvial, eolian, and lacustrine environments. Quaternary deposits not shown at altitudes greater than approx. 205 feet above mean sea level (Suffolk Scarp, in part).

Waccamaw Formation (Tertiary) at surface, covers < 0.1 % of this area

Fossiliferous sand with silt and clay, bluish-gray to tan, loosely consolidated. Straddles Pleistocene-Pliocene boundary.