The prominent unconformity separating Precambrian from Cambrian rocks in the Grand Canyon is the Great Unconformity.
It marks an immense interval when older rocks were uplifted, exposed, and eroded before Cambrian sediments were deposited. In the canyon, Cambrian Tapeats Sandstone rests on much older rocks, including the Vishnu Basement Rocks and formations of the Grand Canyon Supergroup.
The name describes a major gap in the geologic record, not a single perfectly uniform surface. Its age varies by location: the missing interval can represent roughly 175 million years, about 725 million years, or as much as 1.2–1.6 billion years.
A common mix-up is treating every part of the feature as the same technical type of unconformity. Where Cambrian rocks overlie crystalline basement, it is a nonconformity; where they overlie tilted sedimentary rocks, it is an angular unconformity. Together, these exposures are known as the Great Unconformity.