Abstract

Silt- to sand-size clay aggregates are absent or relatively rare in unweathered glacial loesses, but dominant in parna where they resist dispersion in particle size analysis (PSA). There are reports that some aggregates in glacial loesses also resist dispersion. However, one would expect freeze-dried aggregates to have mostly edge-to-face orientation of the platy clays and to be easily dispersed in PSA. We test this hypothesis in this paper. We selected Midwest and Alaskan soils formed in late-Pleistocene loess derived from glacial outwash and floodplains beyond the late-Pleistocene glacial boundary. Analytical methods are those in use by the National Soil Survey Laboratory. Soil A and B horizons dispersed well as shown by the 1.5 MPa water to clay ratios of <0.6. In the C horizon, silt-size, rounded, compound particles (aggregates) were few to common and randomly distributed. A few were volcanic glass. Other aggregates consisted of carbonates or layer silicates. Layer silicate aggregates dispersed well in PSA, as hypothesised; Fe and carbonate cemented aggregates did not. Most of these dispersed when given an ultrasonic dispersion treatment.