Unconformity is the general term for missing pages of Earth history. There are three types of unconformities (below): angular unconformity, nonconformity, and disconformity. An angular unconformity (left) is an erosional surface separating steeply dipping rock layers below from gently dipping layers above. Nonconformity (middle) is an erosional surface separating igneous or metamorphic rocks below from sedimentary strata above. Disconformity (right) is an erosional surface separating horizontal strata below from horizontal strata above and where there is a gap in time.
Exposed in the parking lot of Red Rocks Park & Amphitheater near Denver are red, 300-million-year-old sedimentary rocks resting on gray 1,700-million-year-old metamorphic rocks. About 1.4 billion years of history are missing in the geologic record. This gap is a nonconformity. Questions raised by the absence of strata at Red Rocks are at least partially answered by rocks in other ares of the Front Range. Studying these areas, geologists discovered that the missing strata were indeed deposited at Red Rocks and then later removed by erosion during an uplift associated with mountain building.
Similar nonconformities are also exposed in other places in Colorado. In Glenwood Canyon (pictured right),
sandstone overlies metamorphic and igneous rocks with 1.2 billion years of history missing at the nonconformity. In the Black Canyon of the Gunnison and Colorado National Monuments, sedimentary rocks rest nonconformably on igneous and metamorphic rocks with a gap of 1.5 billion years.
In the angular unconformity pictured left, conglomerate overlies dipping shale in Little Snake Canyon. About 150 million years of Earth history occurred between the deposition of the rocks below and those above the unconformity.
A spectacular example of an angular unconformity in Box Canyon near Ouray is so striking that it often appears in geology texts. The steeply dipping Precambrian strata were originally deposited as horizontal layers and then buried. Later, the layers tilted and folded to a vertical position during a mountain building event and were eventually eroded and truncated. When the vertical Precambrian strata subsided below sea level, the Devonian marine sandstone was deposited on top of them.