The Great Unconformity is found between which geological formations?

A. ~500 million-year-old fossiliferous rocks and glacial sediments
B. 3 billion-year-old gneiss and 2.3 billion-year-old Huronian Supergroup strata
C. 1 billion-year-old gneiss and ~500 million-year-old fossiliferous rocks
D. None of the above

The Great Unconformity exists between 1 billion year old gneiss and ~500 million year-old fossiliferous rocks, representing a gap of about 500 million years in the geologic record, and is due to processes like erosion and non-deposition. The correct option is c.

Explanation:

The Great Unconformity is an iconic geologic feature that marks the boundary between non-fossil-bearing Precambrian rocks (Proterozoic) and overlying fossiliferous Phanerozoic strata.

This unconformity represents a significant gap in the geologic record, which is estimated to be close to 500 million years, as concluded from the angular unconformity seen in places like the Grand Canyon.

To explain why this unconformity exists, one would consider events such as tilting and erosion of older rock units, followed by a long period during which deposition was absent, potentially due to dynamic changes in environmental conditions or tectonic activities. The correct option is c.