English: Mudcracks are diagnostic sedimentary structures indicating subaerial exposure of sediment surfaces, with alternating wet & dry conditions. They are much more common that raindrop imprints, which indicate the same environmental setting. Mudcracks are most likely to occur in fine-grained siliciclastic rocks, fine-grained carbonate sediments, and in some evaporitic successions.

Mudcracks are distinctive and easy to identify, but they do vary in appearance. The cracks themselves vary in width, depth, and areal density. Mudcrack polygons also vary in size & shape.

The examples shown here are developed in modern soil. The brown-colored soil is coated with a thin crust of whitish-colored siliceous sinter. The water that sometimes inundates this surface is geyser runoff, which has small amounts of dissolved silica. Upon evaporation, opal forms. Deposits of opal precipitated from hydrothermal water are called siliceous sinter (also known as opalite). In the splash zone around a geyser itself, the opalite deposit is called geyserite.

Locality: ground adjacent to boardwalk, south-southwest of Giant Geyser, Upper Geyser Basin, Yellowstone, northwestern Wyoming, USA