Lithologic Discontinuities in some Soils on Drumlins:

Theory, Detection, and Application

Randall J. Schaetzl

This paper discusses the importance of lithologic discontinuities in pedologic and geologic research, reviews the primary methods used to detect them, and examines some soils in northern Michigan that exhibit varying degrees of evidence for lithologic discontinuities. Although many different parameters have been used successfully to detect discontinuities in soils, those involving immobile and inert components offer the best likelihood of success, and these data are best reported on a clay-free basis. Parameters involving acquired (pedogenic) characteristics or the mobile element (plasma) of soils should be avoided.

Six Typic Eutroboralf pedons, formed on drumlins, were the primary focus of this study. Obvious-to-subtle evidence exists for lithologic discontinuities within the lower sola of these soils. Frequently, a weakly expressed stone line exists at or near the discontinuity. In this geologically young landscape, the origin of the discontinuity is presumed to have been glaciosedimentologic rather than pedologic (i.e., formed by bioturbation, surface creep, or eolian additions to near-surface horizons).

Depth functions involving clay-free particle-size data, especially for coarser (coarse sand and fine gravel) fractions, were the most consistent indicators of the discontinuity. Mean Particle-size data and heavy versus light minerals were also somewhat useful in discriminating between the two materials. This study may be the first of its kind to use measures of sand grain sphericity (e.g., mean feret diameter, compactness, and shape factor) to identify discontinuities in soils, although the utility of these indices in detecting discontinuities was mixed. The data underscore the need for multiple lines of evidence in the detection of lithologic discontinuities in soils and cautions that they are not all geologic/sedimentologic in origin (Soil Science 1998;163:570-590)