 | near Index, Washington |
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GEOLOGY
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 CO32- and subsequent precipitation of calcite in groundwater causes the concentric growth of calcareous (calcite-cemented) concretions in the sediment.
MINERALS
| CaCO3 - Found as 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. |
Clay-rich concretion |
Clay-rich concretion |
Sandy concretions |
