SUBGLACIALLY REWORKED TILL IN THE PUGET LOWLAND

Seattle is underlain by a complex sequence of glacial and nonglacial deposits from several fluctuations of Pleistocene ice. The physical and engineering properties of most of these deposits have been adequately identified and characterized, and are household names in the geologic community. One deposit, however, has escaped recognition because it is similar to two other commonly encountered glacial units: till and outwash. This is a diamicton that is commonly called "till-like" on boring logs, owing to its till appearance, but yet differs in some of the properties commonly associated with till.

Subglacially reworked till is found on the lee-sides (down-ice) of hills and ridges in Seattle and surrounding areas, leading to the conclusion that it was deposited in a low-pressure or void environment beneath the Puget Lobe. In this environment, sediment that would normally be compacted as lodgement till was subglacially reworked into a jumbled composite of "classic" till, friable till, dirty outwash, and outwash. This results in a deposit with widely varying engineering properties, particularly permeability. Recognition of this deposit as a separate unit is essential for engineering projects and an understanding of Pleistocene glacial processes.