Jackass Mountain conglomerate glacial erratic, Blaine, Washington

By Dave Tucker and Kitty King

December 5, 201o

The H St erratic is about 2 m high. K. King photo. Click to enlarge

Northwest Geology Field Trips subscriber Kitty King of Blaine alerted me to a glacial erratic in Blaine, which recently emerged when brush were cleared along H Street Road. The 2-meter-high rounded stone is one of those wonderful conglomerate boulders from the euphonious Jackass Mountain Formation in southern British Columbia. These distinctive rocks are found in a number of places in Whatcom County. Others that have been profiled on this website are Donovan Rock in Bellingham and the Malloy Village erratics in Ferndale. The Donovan Rock webpage discusses the geology and significance of the Jackass Mountain conglomerate erratics. Among other things, these distinctive boulders demonstrate that the ice that blanketed the Whatcom lowlands originated in the interior of British Columbia, and flowed down the Fraser River valley rather than flowing directly southward out of the BC Coast Mountains.

Your scribe examines the H Street Erratic.

To see the erratic, go 0.5 miles east up the H St hill from the intersection with WA 543 (the truck route to the border crossing). The boulder is in a yard on the north side just beyond Parkview Place. Parking is a problem: best bet my be to turn around at the top of the hill using Allan Street, then come back down and park on the grassy roadside just west of Lincoln Lane. The erratic is on private property, but it can be admired from the roadside. You might want to take along binocs to see the clasts if you are averse to walking up to it.

Detail of the rounded clasts in the erratic show a variety of rocks. The salt-and-pepper granitoids are prominent. K. King photo.

3 Responses

  1. I live in Olympia and have recentlly moved here from Eugene OR. I’ve noticed the erratics here and wondered what their origin was. They mostly seem to be white granite. Canada? Where?

    • Craig,
      Thanks for checking out my website. You asked:
      “I live in Olympia and have recentlly moved here from Eugene OR. I’ve noticed
      the erratics here and wondered what their origin was. They mostly seem to be
      white granite. Canada? Where?”
      The salt-and-pepper granodiorite erratics are most likely carried down from
      the BC Coast Mountains, although there are visually indistinguishable rocks in
      the central and northern Cascades. However, we are pretty sure that glaciers
      flowed UP those valleys from the lowlands, as the main ice lobe advanced quickest in what came
      to be the Puget Lowland during the last Vashon advance.
      I haven’t posted any trips to see big granitic erratics yet, but there is a
      big one down your way that I will write about in the next few weeks. I need to
      make another visit first.
      DaveTucker

  2. This erratic has a mate in the parking lot of the Sevenoaks shopping centre in Abbotsford, B.C. This is a large rock, easily accessible to the public, on the south side of the mall.

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