Existing comment:
The Great Unconformity:
Each layer of rock marks a span of geologic time. But often the layers representing certain spans of time are absent. Either nothing was being deposited at a given time or deposits were later eroded away and left no record of their presence. As a result, young rock may rest directly on very ancient rock. This area of contact between younger rocks and older rocks marks a gap in the geologic record. This area of contact is called an unconformity.
The thin, light tan layer you see on the canyon rim is Entrada Sandstone, a mere 165 to 170 million years old. The rock immediately below it was formed in the Pre-Cambrian period and is about 1-3/4 billion years old, the oldest rocks in the canyon. More than a billion years is absent from the geologic record of Black Canyon! |