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In many places around the world (and through most of the North American continent) Paleozoic rocks have a sharp discontinuity at their base. This widespread erosional surface, called “the Great Unconformity,” often separates layered sedimentary rocks from underlying metamorphic and igneous rocks (the so called “crystalline basement”). In this photo taken near Manitou Springs, CO, USA, the Great Unconformity is marked by the dashed line and overlain by the Cambrian Sawatch Quartzite. Hammer (encircled) for scale.
The flat erosional surface separating gray clays (Argille Subapennine Fm., Pleistocene) from yellowish sands is called ravinement surface and was cut by waves in shallow water. Erosional surfaces in the rock record are essential for reconstruction of sea level and tectonic changes. They help unraveling the history of our planet but also illustrate the power of geologic processes. Photo taken near Canosa, Puglia, Italy.
A "nonconformity" is a type of discontinuity surface where sedimentary rocks lie above crystalline rocks. The dashed line in the picture marks a nonconformity between the Mesoproterozoic Pikes Peak Granite (bottom/right) and the Pennsylvanian conglomerates of the Fountain Formation (top/left), near Denver, CO. The contact in this outcrop (near Red Rocks Amphitheater) is marked by a plaque.