Fossil hunting in Juneau

just look for the Gastineau Formation

My two companions scrambled up the soggy slope with agile ease, and I lumbered along behind. My main job, as usual, was to throw sticks for the four-footed friend, but our real mission was finding fossils. Around Juneau, we don’t expect to find dinosaurs, ichthyosaurs, or mammoths, such as those found up north. Nor do we find fossil remains of bears, caribou, and foxes, such as those in Prince of Wales caves. Nope, nothing quite so spectacular.

However, we can find lots of invertebrate fossils: shells of clams, snails, barnacles, cases of certain polychaete worms, bits of sea urchin shells, and—for the really persistent, dedicated, and knowledgeable searchers—microfossils, identifiable only under a microscope, including a great diversity of foraminifera. “Forams” are amoeboid protozoans that make sometimes elaborate shells, usually of calcium carbonate. Some live as plankton but they are reportedly more often associated with sediments.

On this day, our search for shells was almost immediately successful. A tiny rivulet ran down the forested slope and we followed its course upstream. Eventually we came to an even smaller side drainage that was little more than a seep. Along the sides of the seeping area, near the confluence with the rivulet, the mud had sagged and slumped, and there—poking up out of the gooey mess—was an intact snail shell. Whoopee! Picking through the muck yielded several broken clam shells too, and we felt amply rewarded for our small efforts.

These shelly fossils are found in what’s known as the Gastineau Formation, named (of course) for our local channel, where it is commonly observed (although similar formations are known farther south). The formation is typically composed of variable proportions of fine silt, sand, gravel, and sometimes cobbles, deposited on and along marine beaches during and after the last principal glaciation. Some glacial material was added as it washed down from the big ice or was carried around by bergs. The known ages of the deposits range in the vicinity of ten to twelve thousands years ago.

As sea levels rose (relative to the land, which also began to rise) when the big ice retreated, beaches were formed and inhabited by a great diversity of invertebrates; as sea levels fell (again relative to the rising land), the beaches were abandoned by the sea and many of their inhabitants were stranded and died, leaving their hard parts in the sediments. One list of the diversity of animals found in the Gastineau Formation includes at least twenty kinds of clams, about eighteen kinds of snails, and dozens of kinds of forams, plus a variety of other creatures.

ancient-beach-shell
Clam shell from the Gastineau Formation. Photo by Katherine Hocker

The Gastineau Formation is exposed at numerous locations in Juneau: along road cuts, stream cutbanks, slumping slopes, construction sites. It occurs at three levels, up to about 750 feet above present sea level, showing that beach formation and abandonment happened several times. The layers of the formation vary in thickness, from just a few feet to several tens of feet in depth. Composition and density of the sediments differ among the levels and among locations where the formation is exposed. Of course, the composition of the invertebrate fossil fauna varies too, and researchers can tell, by the nature of the sediments and by the relative abundance of different species of fossil, what the conditions were like on these abandoned beaches—if this was a place of quiet water or turbulent waves, of brackish or salt water, of very shallow or somewhat deeper water, and so on.

For most of us, the invertebrate fossils are not as exciting as the big stuff. But they are interesting and instructive all the same, providing insight into changes in our so-changeable landscape.

Thanks to a friendly local geologist for useful references.

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