DETAILED STUDY OF THE VASHON TILL FROM THE PUGET LOWLAND, WASHINGTON, THROUGH MICROTEXTURAL ANALYSIS OF QUARTZ SAND GRAINS

The Vashon Till, a late Pleistocene unit, covering the greater portion of the South Central Puget Sound in Western Washington State, is a typical, moderately homogenous very poorly sorted glacial deposit. The sand fraction of the deposit is composed of lithic and feldspar fragments, and more than fifty percent quartz grains. From the fine sand portion of the sample, quartz grains were hand selected under a light microscope and mounted for study using a scanning electron microscope. Microtextures on the grains were studied to determine the assemblage of features found within the deposit, enabling comparison with other typical glacial deposits as well as those from other depositional environments. The Vashon grains display a high occurrence of sharp breakage features, with high relief, and numerous deeply incised conchoidal fractures. Along with abrasion features, these microtextures are indicative of a crushing environment, as was expected from the till. The microtextures identified within the Vashon till are fairly similar to those outlined in previous studies conducted on glacial quartz sand grains. To further outline the unique qualities of this till, comparisons were made with quartz grains found within other active environments including: desert dunes, back beach dunes, fluvial, and beach environments. As many microtextures can be formed in multiple environments, there was moderate overlap between each environment. Individual microfeatures are not independently suggestive, however, the grouping and frequency thereof provide strong evidence for the environment of deposition, and can serve as unique signature for a deposit. The data collected on the Vashon till may be useful in future attempts to distinguish between interglacial and glacial deposits within the Puget Basin, where other petrologic evidence is limited.