Silcrete is a hard, grey, sedimentary rock made of silt-sized detrital quartz grains in a matrix of interlocking microcrystalline quartz. Silcrete beds occur in North Dakota in the Rhame bed, which is used locally to mark the top of the Paleocene-age Slope Formation. Silcrete beds also occur just below the Cretaceous-Tertiary (K/T) boundary (lithostratigraphically determined), at the K/T boundary, and at various horizons in strata of Paleocene age throughout the Western Interior. Silcrete beds are also found worldwide in association with the K/T boundary and Paleocene-age rocks.

Silcrete deposits were first recognized in North America, near Rhame, North Dakota, by the writer in 1978. Dr. William Clemens (University of California at Berkeley), who the writer met while visiting Marshall Lambert in Ekalaka, Montana, suggested a review of the Australian geologic literature because the siliceous rock in this area looked like silcrete he had seen in Australia.

The exact origin of silcrete is still controversial, but it is generally accepted to be part of a paleosol, indicating a stable climate continuous over a significant length of time. Little mention or acknowledgment of silcrete is made in modern paleosol studies, although it is part of the calcrete-silcrete-ferricrete duricrust continuum. Silcrete is significant in many ways in that it 1) has a distinctive and recognizable lithology; which is similar in appearance wherever it is found; 2) can be traced over large areas, even using air photos, making it a mappable unit; and 3) may prove to be an important chronostratigraphic marker associated with the K/T boundary, similar to the use of the iridium-enriched layer. Additional study of silcrete deposits at the K/T boundary may provide geologists with another important tool in interpreting events at this time.