That it seems to be expressed in Ice Age (16 Ka) deposits implies a very recent but entirely unknown event; but perhaps these recent deposits are only draped over a much older topography. A recent offset might explain the apparent offsetting of north–south glacial drumlins bisected by the Ship Canal, but is not evident in more eastern segments. Alternately – and this would seem very pertinent in regard of the OWL – perhaps some mechanism other than strike-slip faulting creates these lineaments. Seattle Fault A locally notable feature that crosses the zone of the OWL is the west–east Seattle Fault. This is not a strike-slip fault, but a thrust fault, where a relatively shallow slab of rock from the south is being pushed against and over the northern part. (And over the OWL.) One model has the slab of rock being forced up by some structure about 8 km deep. Another model has the base of the slab (again, about 8 km deep) catching on something, which causes the leading edge to roll. The nature of the underlying structure is not known; geophysical data does not indicate a major fault nor any kind of crustal boundary along the front of the Seattle Fault, nor along the OWL, but this could be due to the limited reach of geophysical methods. Recent geological mapping at the eastern side of the Seattle Fault suggests a decollement (horizontal plane) about 18 km deep. These models were developed in study of the western segment of the Seattle Fault. In the center segment, where it crosses surface exposures of Eocene rock associated with the OWL, the various strands of the fault – elsewhere fairly orderly – meander. The significance of this and the nature of the interaction with the Eocene rock are also not known. Examination of the various strands of the Seattle Fault, particularly in the central section, is similarly suggestive of ripples in a flow that is obliquely crossing some deeper sill. This is an intriguing idea that could explain how local and seemingly independent features could be organized from depth, and even across a large scale, but it does not seem to have been considered. This is likely due, in part, to a paucity of information on the nature and structure of the lower crust where such a sill would exist. Southern Whidbey Island Fault and RMFZ The Southern Whidbey Island Fault (SWIF),