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    Northwest Geology Field Trips, by Dave Tucker, is licensed under a Creative Commons Attribution-Noncommercial- Share Alike 3.0 United States License. You can use what you find here, repost it with attribution to the author, "remix" it for your own purposes, but may not use it with the intent of making money off of it.

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Error on latest Jackass erratic report

The Federal Way Jackass Group conglomerate erratics I reported on earlier today are about 6 miles further south than the purported one at Des Moines Beach Park. So, if you wish to document that one, it wouldn’t be further south, but still worth knowing about. I have corrected the original post.


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.

Another southern Jackass erratic sighting

Jackass conglomerate erratic tucked among logs on Camano Island. Photo courtesy Carl Johansen and Frank Alishio. Click to enlarge.

Website subscriber Carl Johansen sends these photos of erratics from the Jackass Formation conglomerate on the beach on the east shore of Camano Island. Unfortunately, the beach is private. Earlier this winter, I asked people to keep their eyes open for any of these distinctive erratics south or west of Whatcom County. This is relevant, as they have been associated only with the last gasp of Pleistocene glacier readvance (the Sumas advance) which barely reached Bellingham and was entirely on land- no glaciomarine component is recognized, so they didn’t flow to their current resting places in calved icebergs. The presence of these distinctive boulders to the south or west negates that theory, put forth by Dr. Don Easterbrook of WWU. Thanks to the reports sent to this website, it looks like these rocks can not be used to delineate the extent of the Sumas readvance.

I’m still hoping for photos of a reported Jackass erratic way down south in Des Moines.

Verified Jackass erratics. Red line marks approx terminus of Sumas readvance.

A smaller Jackass erratic, north of the first. Photo courtesy C. Johansen and F. Alishio.