The dashed yellow line marks the unconformity that Hutton observed at Siccar Point in Scotland.
Sometimes rocks have layers that are clearly missing. These breaks in the rock record are called unconformities. Unconformities are caused by a long period during which deposition ceased; rock was then removed by erosion, and deposition was then later resumed. It was just such an unconformity at Siccar Point in Scotland that reinforced Hutton’s views in 1788 about the rock cycle. (See C2: The Rock Cycle – Further Connections – History Link for more details.)
Did You Know?
The Great Unconformity is a massive unconformity that is found in rock layers nearly everywhere across the globe. In the Grand Canyon, it represents a time gap in the geologic record stretching from 250 to 1,200 million years ago.
The solid yellow line represents the Grand Unconformity in the Grand Canyon.