University of Washington Publications in Geology The Geology of the San Juan Islands

SEDIMENTARY ROCKS

GENERAL CHARACTER

The sedimentary rocks exposed on the San Juan Islands range front middle Paleozoic to Recent in age.

The Paleozoic rocks belong to one more or less conformable series, known as the San Juan series.¹

¹McLellan, R. D., The Devonian Orcas Groups of Washington: Amer. Jour. Sci., 5 Ser., vol. 8, p. 217, 1924.

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The lower portions of the San Juan series are composed largely of cherty quartzite and argillite with scattered lens-shaped deposits of crystalline limestone. The upper portions of the series have a more varied lithology, but tuffaceous graywacke, argillite, schist, and conglomerate are the most abundant rock types. Almost all of the Paleozoic rocks are highly metamorphosed.

On Vancouver Island there is a series of rooks known as the Sicker series² which is apparently unfossiliferous. It is suspected by the writer that the Sicker series and the San Juan series are identical, even though the former has been provisionally correlated with the Jurassic.

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²Clapp, C. H., Southern Vancouver Island: Geol. Survey Canada, Summary Report, 1909. pp. 88-89, 1910.

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The Mesozoic sedimentary rocks occurring on the San Juan Islands belong to the Triassic, lower Cretaceous, and upper Cretaceous systems. They are composed chiefly of conglomerate, arkosic sandstone, and shale. Mesozoic limestone beds are small and quite rare in this region. The Mesozoic rocks, particularly those belonging to the upper Cretaceous system, are only slightly metamorphosed. The Tertiary rocks are composed of unmetamorphosed arkosic sandstones and conglomerates of brackish or fresh water origin. The Tertiary and late Mesozoic rocks have apparently been subjected to the same period or periods of folding.

During the Pleistocene period the glaciers cut deeply into the older formations and, on retreating, left a thick mantle of glacial drift on many of the islands.