shales and wackesInterbedded shales and wackes in sedimentary rock of the Silurian Period, Lower Silurian Aberystwyth Grit Formation, Wales.
sedimentary rock, Rock formed at or near the Earth’s surface by the accumulation and lithification of fragments of preexisting rocks or by precipitation from solution at normal surface temperatures. Sedimentary rocks can be formed only where sediments are deposited long enough to become compacted and cemented into hard beds or strata. They are the most common rocks exposed on the Earth’s surface but are only a minor constituent of the entire crust. Their defining characteristic is that they are formed in layers. Each layer has features that reflect the conditions during deposition, the nature of the source material (and, often, the organisms present), and the means of transport. See also sedimentary facies.
