formed rocks, and then deposition resumed. In each case uplift and erosion are followed by subsidence and renewed sedimenta tion. Unconformities are important features because they repre sent 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 motakea nip through time. The canyon's colorful strata record long history of sedimentation in a variety of environments- tidal flats, and sand dunes. Butthe 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 11.7isa geologic cross section ofthe Grand Canyon. Refer to as you read about the three basic types ofunconformities: angu lar unconformities, disconformities, and nonconformities.