Unconformities
When we observe layers of rock that have been deposited essentially without interruption, we call them conformable. Particular sites exhibit conformable beds representing certain spans of geologic time. However, no place on Earth has a complete set of conformable strata. Throughout Earth history, the deposition of sediment has been interrupted again and again. All such breaks in the rock record are termed unconformities. An unconformity represents a long period during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed. In each case uplift and erosion are followed by subsidence and renewed sedimentation. Unconformities are important features because they represent significant geologic events in Earth history. Moreover, their recognition helps us identify what intervals of time are not re resented by strata and thus are missing from the geologic record. rocks exposed in the Grand Canyon of the Colorado River represent a tremendous span of geologic history. It is a wonderful place to take a trip through time. The canyon's colorful strata record long history of sedimentation in a variety of environments- tidal flats, and sand dunes. But the advancing Seas, rivers an record is not continuous. Unconformities represent vast amounts of time that have not been recorded in the canyon's layers Figure 11.7 is a geologic cross section of the Grand Canyon. Refer to as you read about the three basic types of unconformities: angular unconformities, disconformities, and nonconformities.