BOUNDARIES OF SOILS USED AS CONTACTS FOR EOLIAN PINEHURST FORMATION, PATRICK, S.C. (1:24,000) GEOLOGIC MAP

The recently completed geologic map of the Patrick, S.C. (1:24,000) quadrangle contains two extensive formations – the Middendorf (K) fluvial deposits formed of kaolin-rich quartz sands and clay beds, and the Pinehurst (Q) eolian dunes and sand sheets. Stream watersheds carved into the deeply weathered Middendorf sediments display nearly 100m of relief. However more than 80% of the area is mantled by eolian sand sheets, parabolic dunes, and transverse dunes, some of which have parallel linear crests spaced 200m apart. GPR data reveal dunes can exceed 6m thickness. The extents and thicknesses of the largest sand bodies should not be ignored on the geologic map, and thus deserve a formal cartographic designation. However the margins of dunes draped over stream interfluves are indistinct; accurate placement of contacts would require many hand auger borings to establish both the presence of thick sand and the boundaries of sand sheets and sand dunes in rolling topography. Fortunately Morton (1995) mapped two soil series in Chesterfield County that marked the edges between dunes and sand sheets. The Alpin sand, mapped across 28% of the quadrangle, consists of eolian fine sand more than 3m thick; a thick pale brown (10YR) argillic Bt horizon (>10% clay) tops this soil profile. The Condor sand, 11% of the area, consists of at least two different sheets of eolian fine sand that together exceed 2m thick. Two brown (10YR) Bt horizons occur in this profile, one marking the top of the buried sand bed. Other soil series (e.g. Ailey, Troupe, Vaucluse – 28% of area) formed in eolian sand less than 2 m thick lie over red-orange (10R) weathered Middendorf sediments. Because these soil data were available, and wherever checked the soil series and boundaries were properly identified, we decided to portray the Pinehurst formation where it is >2m thick and to use the boundaries of the Alpin and Candor soils as approximate formation contacts.