THE GEOLOGIC SOIL-ASSOCIATION MAP OF PENNSYLVANIA

Soil is the ultimate in-situ weathering product of parent material, either bedrock or derived surficial materials, e.g., colluvium or till. Characteristics of a soil series, the smallest, most unique soil subdivision, relate directly to underlying parent material, particularly its texture and mineralogy. On detailed soils maps, generally at 1:20,000 scale, the multitude of related series and the smallness and irregularity of individual mapped series units make them seem remote from larger, more regularly-shaped, mapped rock units. However, associations formed of closely-related series, a common summary generalization shown as a map in county soils reports, have more regular outlines and show a close map relationship to mapped parent materials.

The Geologic Shaded-Relief Map of Pennsylvania (GSRM) (Miles, 2003, 1:500,000 scale) has 139 map units. These map units result from numerous combinations of separately named but lithologically similar rock units shown as 200+ separate units on the Geologic Map of Pennsylvania (Berg and others, 1980). Evaluation of general descriptions and soil profiles from county soils reports verifies an excellent correlation between the areal distribution and lithological character of the GSRM units and the parent materials of soil associations on county soil-association maps for PA’s 67 counties. Study of this information led to creation of the Geologic Soil-Association Map of Pennsylvania (GSAM).

The GSAM has 72 soil associations composed of 99 soil series and 3 non-soil units. Some of these associations are new assemblies created for the GSAM. Of the 99 series, 56 occur in only one association, while one, Hazleton, occurs in 9 associations. Hazleton develops on sandstone on convex hilltops and hillsides, a common bedrock occurrence in PA.

Boundaries on the GSAM correspond almost exactly to those on the GSRM, but a single association on the GSAM may cover an area that includes several, adjacent, lithologically similar but separately mapped, GSRM rock units. For example, the Buchanan-Laidig-Hazleton association covers an area underlain by Tuscarora, Juniata, and Bald Eagle Formations and the Clinton Group, all sandstone-rich units.