Update on the Exotic Erratic at Leschi Park, Seattle

In late February I posted an article about a fossil-rich erratic at Seattle’s Leschi Park, described by Darryl Howe.

Here is an updated report, by Doug McKeever, Whatcom Community College geology instructor. Check out Doug’s fun website here.

The Nooksack Gp rocks are the polygons labeled 'N' in this clip from the Mount Baker 30 x 60 USGS map. Mount Baker is at left center. Skagit valley is the yellow area running across the bottom.

I briefly visited this erratic on March 6. It is clearly visible directly west and uphill in the woods (recently cleared of most undergrowth by trail crews) from the small parking lot by the tennis courts.  I took pictures but they don’t show anything that Darryl’s don’t show.  It is indeed “chock full” of bivalve fossils.  I am not enough of a paleontologist to know them, nor could I recognize the formation from which the erratic came.  It looks similar to Nooksack Group rocks I have seen in the Mt. Baker area….it is fine-grained (very fine sand or silt) and appears to be well sorted and not particularly well stratified.  The rock is weathered, as one would expect from sitting in the woods in a humid climate for about 13 or 14,000 years since being dumped by ice.  Curious folks have hammered at a few chunks that are lying nearby.  This is a fascinating find, the likes of which I haven’t seen among several thousand erratics personally observed from the Fraser Glaciation. It  is worth further investigation by paleontologists (George Mustoe, are you reading?) and sedimentary petrologists.   I wonder if anyone from UW has seen and described it.

DT adds: The Nooksack Gp. rocks are known from the vicinity of Mount Baker; not all are fossiliferous.  The best known fossil localities are from the argillite at Fossil Creek (south of Church Mountain) and in bedded siltstones on Chowder Ridge, at the south end of Skyline and Cougar Divides. If this erratic is from the Nooksack Group, it would have been carried out the Skagit (or conceivably via the Nooksack) and into the main stem Vashon ice covering the future Puget Lowland. That is at least 95 miles in a reasonable path via the Skagit.


One Response

  1. Buchia from the Harrison Lake area? The host rock sure looks like the Harrison Lake material. The Nooksack Group material tends to run darker with some iron stain.

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