In the Grand Canyon, there is seemingly 1.2 billion years of rock and thus geologic history missing. As, 520 million year old rock directly overlies 1.7 billion year old rock with nothing inbetween the two. This is what is referred to as the "Great Unconformity", and happens to also occur in states such as Colorado, Nevada, and New Mexico. This video will discuss one possible reasoning behind why 26% of Earth's geologic history is missing across a vast area.