NORTH FORK of the SKYKOMISH RIVER near Index, Snohomish County, Washington

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LOCALITY IMAGES



GEOLOGY
North Fork of the Skykomish River - Holocene glaciolacustrine deposit (glacial lake) exposed at a cutbank along the Skykomish river. The deposit is comprised of massive dark gray clay interbedded with mm-scale sand layers. Dropstones are common. Diffusion of Ca²⁺ and CO₃^2- and subsequent precipitation of calcite in groundwater causes the concentric growth of calcareous (calcite-cemented) concretions in the sediment.

MINERALS

Calcite-

CaCO3 - Occurs as main mineral component within concretions. Calcite is the cementing agent, filling the pore space in clay and sand sediment layers. Diagenetic calcite precipitation in sediment pore space produces light-gray, flat, rounded nodules that weather out of the hillside. Fine-grained clay-rich concretions are commonly symmetrical. Sand-rich concretions are typically irregular in shape. Concretions range in size from the size of a pea to six inches, with most in the one to three inch range.

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Clay-rich concretion cemented with CALCITE	Clay-rich concretion cemented with CALCITE	Sandy concretions cemented with CALCITE