An unconformity is a burried erosional surface. That is, soil and rock that was once present was erroded over a very long period of time by wind and rain. This process still happens today. The mountains that surround Las Vegas may some day, millions of years from now, be flat ground or gently rolling hills.

This unconformity was first identified by a geologist named Clarence Dutton in 1882. He found it in the layers of rock at the bottom of the Grand Canyon. Dutton probably named it "Great" because it was in the Grand Canyon. He certainly didn't know how very old the bottom rocks were or how much time it took for errosion to occur and new rock to be deposited there, because there was no way to get an absolute date (as apposed to relative date) on the rocks.

Much later on, geologists finally developed ways to figure out the absolute age of rocks. Geologists used radiometric* dating to find out that the granite and schist rocks are 1.7 billion years old (1,700,000,000)! The sandstone rock is 550 million years old. That means that about 1.2 billion years of the earth's history eroded away before the sand that made the sandstone was deposited. 1,200,000,000 years! Wow, that's a lot of time! Now we can say it really is a "Great" unconformity.

*Radiometric dating has to do with how unstable some elements' atoms are. Sometimes they loose parts and become a different kind of element. Scientists compare the ratio of the parent element to the daughter element and can figure out how old a rock is.