• This bedrock map of North America illustrates one major unconformity where the crystalline rocks of the Canadian Shield (red) are overlapped by the fossil-bearing rock formations of the North American plains (green). (Map: USGS/Wikimedia Commons)

Unconformity
[uhn-kuh n-fawr-mi-tee]

Definition
An interruption between layers of rock, in which the upper layer is much younger (even by more than a billion years) than the lower layer.

Origin
The first known use is believed to date to the 17th century. Its first geological use is from 1829.

Example
A bedrock map of North America (shown above) nicely illustrates one major unconformity (part of the world’s “Great Unconformity”) where the crystalline rocks of the Canadian Shield (red) are overlapped by the fossil-bearing rock formations of the North American plains (green). Not coincidentally, a number of Canada’s largest lakes — Great Bear, Great Slave, Lake Winnipeg and Georgian Bay — lie along this transition zone. Significant mineral deposits are also often found near unconformities.