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Glaciers Versus Rivers
After the Yellowstone Caldera erupted 600,000 years ago, the canyon area was sheathed in a blanket of lava flows. Ancient faulting in the area accelerated the erosion process of these flows, and glacial deposits excreted during the Ice Age were also swept away through erosion.
Major sculpting activity, however, did not occur until 14,000 to 18,000 years ago at the conclusion of the last Ice Age. Several glaciers situated at the outlet of Yellowstone Lake began to melt as temperatures started rising. The natural ice dam consequently broke, and a flood of water surged downstream in numerous flash flood occurrences. The constant release of water created a continual erosion force that cut out the classic V-shaped valley forming the canyon. Such shaping provides geological testimony to running water/river erosion. To this day, the Yellowstone River continues to carve the canyon with its natural erosion properties.
Although researchers are unsure of its formation date, a geyser basin once filled the canyon area below the Lower Falls. Lava flows, underground hot spots, and fault lines created the extensive geyser basin. The basin spurred a few hot springs and geysers into existence, many of which are visible to this day. Geologists speculate that nearby Clear Lake is also a product of this early geyser basin.
Thousands of years of hydrothermal activity has altered the canyon’s natural mineral compounds, causing unique shades of red, yellow, gold, and orange to bleed down the canyon walls. The chemical alteration in the canyon walls dates back to the area’s old geyser basin. Heat from the basin essentially baked the rock and created chemical changes in the canyon’s iron deposits. As a result, when the transformed rocks were exposed to air and water, they began to oxidize and rust. These rocks continue to oxidize and add new coloring on an annual basis.