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Federal Way- furthest south Jackass conglomerate yet

By Dave Tucker

Federal Way's Jackass conglomerate boulder. Click to enlarge. Courtesy Neil Gilham

Neil Gilham sent these photos taken in Federal Way. The erratics sure look like conglomerate of the Jackass Group. Neil thinks that given the cost of moving these from elsewhere, these were probably dug up on site during the construction of these apartments. I don’t have the exact location, but they are on private property.

This is the furthest south the conglomerate has been photographed. There are reports of another Jackass on the beach at Des Moines Beach Park somewhere north of the foot bridge across the Des Moines Creek. Being on the beach, it is most likely also in situ. Maybe someone can go for a stroll  with a camera, find it, verify the report. Google Earth shows an 8 foot boulder on the beach only 75 yards north of the Park’s footbridge. Is that it? Note that this purported Des Moines Jackass is about 6 miles north of the ones pictured in this post.

Jackass conglomerate boulder detail. Click to enlarge. Courtesy Neil Gilham

Participation by all the citizen geologists in this ‘find the Jackass’ project have disproven beyond reasonable doubt a hypothesis that Jackass conglomerate erratics are generally indicative of the extent of the final,  Sumas readvance of Fraser ice. (See the December 26, 2010 comment by Don Easterbrook following the story about the Saar Creek erratic). That advance is shown by mapping and LiDAR evidence to have made it only as far south as Bellingham.


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