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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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Jackass conglomerate erratics on Orcas Island.

A Jackass on the beach.

Alert reader Michael Yeaman of Eastsound, Orcas Island, has sent photos of Jackass Group conglomerate erratics on lovely Crescent Beach, just east of Eastsound. Recall I sent out a call for citizen scientists to be on the lookout for these- read that request here. The idea is to show whether these erratics are strictly indicative of deposits of the final phase of the Fraser glaciation, the ‘Sumas’ readvance, or whether they came earlier, with the ‘Vashon’ advance. Michael’s photos show erratics are just like the one’s Adena sent (January 3) from near Port Townsend. It is clear from the photos received so far that the latter must be correct, as Port Townsend and Orcas Island are well beyond the maximum extent of Sumas ice. I have heard from a couple of other readers, that there are suspected Jackass erratics further south. If you have a photo and location, please send it in.

No doubt, this is Jackass conglomerate, from BC, carried by ice to Orcas Island.

Cheers, Dave

Crescent Beach and Eastsound, Orcas Island. Click to enlarge. Google Earth image capture.

2 Responses

  1. The shoreline bluff to the east of this site is underlian by glacial marine drift so it is possible that this particular erratic was floated to the site via a ice berg.

    • Yes, of course, Dan, but if so, it didn’t float here during the Sumas readvance, which was not associated with a glaciomarine drift, but rather from ice breaking up as the much bigger, and earlier, Vashon lobe was receding as it was floated on the influx of water via the Juan de Fuca Straits. The point is, these Jackass boulders are not very indicative of the Sumas stade, as Don Easterbrook maintains in this comment: “Most of the Vashon erratics are granitic, most of the Sumas are Jackass conglomerate” found near the end of the comment thread here.

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