Over the last 300,000 years there were two major periods of glaciation: The Bull Lake Glaciation period occurred from 300,000-127,000 and the Pinedale Glaciation Period occurred from 30,000-12,000 years ago. Another period of uplift and erosion during the Tertiary period raised the Rockies to their present height and removed significant amounts of sedimentary deposits and revealing the much older basement rocks. This process uplifted the modern Rocky Mountains, and was soon followed by extensive volcanism ash falls, and mudflows, which left behind igneous rocks in the Never Summer Range. Starting 75 million years ago and continuing through the Cenozoic era (65-2.6 Ma), the Laramide Orogeny (mountain-building event) began. This flooding left behind large amounts of sedimentary deposits, like the Pierre Shale and Fox Hills Formation (sandstone). 100 million years ago the entire state of Colorado and much of middle North America was submerged under the Western Interior seaway. Over the next couple hundred million years the ancient Rockies eroded away, leaving behind sediment and a much less rugged landscape. This ancient mountain range was much smaller than the modern Rockies, only reaching up to 2,000 feet high and stretching from Boulder to Steamboat Springs, Colorado. At about 285 million years ago, a mountain building processes raised the ancient Rocky Mountains. This caused regional metamorphism and created the basement igneous and metamorphic rocks found within the park.ĭuring the Paleozoic era (544-245 Ma), inland seas covered much of present-day North, depositing thick layers of marine sediments that would later turn into sandstone and limestone. Shortly after that, relatively speaking, at 1.6 billion years ago a large volume of magma pushed into the older rock creating what is known as the Boulder Creek Batholith. 1.7 billion years ago, during the Precambrian Era, the oldest metamorphic rocks (such as schist and gneiss) were being formed.
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