A new study led by CU Boulder reveals the complex history behind one of the Grand Canyon’s most well-known geologic features: A mysterious and missing gap of time in the canyon’s rock record that covers hundreds of millions of years.The research comes closer to solving a puzzle, called the “Great Unconformity,” that has perplexed geologists since it was first described nearly 150 years ago.Think of the red bluffs and cliffs of the Grand Canyon as Earth’s history textbook, explained Barra Peak, lead author of the new study and a graduate student in geological sciences at CU Boulder.