Tuesday, February 8, 2011

The Puget Ice Lobe Turns Up a Valley

Drumlin features marking ice flow paths and the Devil's Mountain Fault Zone

Deep snows in the Canadian Coast Range evolved into massive glaciers that flowed south into lowland between the Cascade Range and the Olympic Range during the last glacial period. The LiDAR image above from southeast of Mt. Vernon captures the flow path that was generally from north to south. But note the drumlin features that make a turns towards the east. It is a bit counter intuitive, but during the maximum extent of glacial ice in western Washington, the glaciers in the Cascade range did not extend down to the Puget ice lobe. The ice flow in the lower valleys was up the valleys from the thick ice in the Puget lowland.

The Devil's Mountain fault zone is a fairly major structure and though no clearly distinctive offsets are present in post ice age deposits, it is hard to imagine that this fault is not active and deformations and hints of offset on the north end of Whidbey Island suggest that this fault is capable of giving Mount Vernon and the area a good shake.

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