By Dave Tucker; science contributions by Doug McKeever and Wes Gannaway
Original heads up and photos by Darryl Howe
An interesting fossiliferous erratic has been found following recent trail work in the upper part of Leschi Park in Seattle. Location is along a new section of trail just west of Lake Washington Blvd and south of Yesler Way. 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. The rock is not huge, but one of the more intriguing ones I’ve heard of since sending out the ‘cool erratic call out’ to website visitors. The erratic is chock full of bivalve, perhaps brachiopod, fossils. It looks similar to Nooksack Group rocks that Doug and Dave have seen in the Mt. Baker area. The fossil content also argues for a source in the Harrison Lake area of southern BC. We expound further about these potential source areas below. The rock 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 none of the authors have seen among several thousand erratics personally observed from the Fraser Glaciation. It is worth further investigation by paleontologists and sedimentary petrologists.
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 most likely have been carried down the Skagit (or conceivably but less likely 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. It may have floated here in an iceberg as the ice deteriorated and receded northward.
Wes Gannaway, a paleontologist in Bellingham, suggests the source area for the bivalve-bearing Leschi erratic could be the rocks around Harrison Lake, BC. He has only seen the photos posted here of the fossils in the erratic, so this is remains a hypothesis only, but it is a reasonable one.
Wes wrote: “The gray siltstones are along the shore of Harrison Lake and contain
an assemblage of ammonites and clams including the Buchia crassicola, which
I believe are the dominant Buchia species at Fossil Creek in the Nooksack
Group. The formation borders on the Cretaceous/Jurassic boundary, age
equivalent to the Nooksack Group. The fossils are abundant in the Mysterious
Creek Formation. C.H. Crickmay wrote a paper in 1930 on the locality. His
paper names them Aucella sp. and Jeletzky named them Buchia, although
I don’t know enough about the species to identify which one is in the
Leschi Park boulder. Aucella is also found in the Jackass Mountain area. Someone with
the knowledge of the species should take a look at the fossils in the
boulder. I am just looking at the photo and using the general shape and rock
color to call it Buchia from Harrison Lake.”
Thanks for that, Wes. Here’s more information: (ain’t the web wonderful?)
Here’s another blogger’s post about fossil hunting in the Mysterious Creek Formation. And this blog has photos of large Buchia fossils, including this one.
Further web investigation turns up this abstract of a 1983 UBC MSc thesis by Andrew Arthur on stratigraphy and fossils in the Harrison Lake area. If you follow the above link to the thesis abstract, you’ll find a download file of the entire thesis as a pdf. In it are many photos and drawings of fossils, mostly ammonites, found in the Mysterious Creek and related formations.
As a source region for the Leschi erratic, the Harrison Lake area makes good sense. The Vashon ice flowed south right through there. George Mustoe told me that he picked up a Buchia-laden erratic on the beach at Larrabee Park back in the 1960s. That surely would not have come from the Nooksack Group, although the correlative rocks at Harrison Lake would be a reasonable candidate.
Any paleontologists out there who want to take a crack at this problem?
If anyone has more photos or insights into the origin of this unusual erratic, please write to: