Boring Clams Lend an Otherworldly Appearance to a Miocene Shelf

An alluvial miocene formation shaped by changing sea levels, wave action, and boring clams has an otherworldly appearance.


The seaward part of the headland, in the foreground, is Angora Peak sandstone. Composed of well-cemented particles, this resistant shelf once hosted a multitude of boring clams. You can still see shells in some of the the holes. The vertical cliff in the center background is basalt from an ancient lava flow.

Remnant boring clam shell in miocene sandstone

I can’t help wondering if the remnant shells are modern, or traces of a past form.


Alan R. Niem. 1975. GEOLOGY OF HUG POINT STATE PARK NORTHERN OREGON COAST. The Ore Bin, State of Oregon Department of Geology and Mineral Industries, Vol. 37, No.2: 17-36.

Out of This World


  1. I have the same problem with deciding how old the boring clam shells are. I have encountered them often in various contexts. The shells look so perfect that it is difficult to believe that they are as old as the rocks into which they bore. I wonder if anyone has thought of the feasibility of radiocarbon-dating the shells. It might be possible. Costs of doing it must have come down considerably since I had some archaeological oyster shells dated several decades ago.

    1. I saw a post on Twitter of “~80,000 yr. old pholadid bivalves still in their borings on an ancient marine terrace surface (marine isotope stage 5a), Año Nuevo State Park, CA” the terrace, and the shells (though smaller and more delicate) looked similar to what I showed in my post. So, somebody is doing it. The post was by Seth Finnegan, a prof at UC Berkeley. Makes me think 80,000 years is possible. Not super old, but still astounding.

  2. They do lend an otherworldly appearance to the shelf. I was led to believe that boring clams were native to the Indo-Pacific region. Are modern ones also common to the Oregon coastline?

    1. Your question got me thinking. Thanks! We have pholadids (piddocks), and a few other borers on the west coast, US. In Oregon, where I live, the flat-tip piddock, Penitella penita, is abundant when the rocks they like are present. They used to be harvested for food, and still are to some extent, but I’ve never seen it.

      1. I had to look up Penitella penita for images and found one you had posted previously. I believe I have seen the shells in Southern California without realizing what they were. Thank you for the information. It was enlightening.

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