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Geologic Cross Section of the San Luis Valley
The Rio Grande Rift, one of the most notable geologic forces
in the region, extends northward from Mexico into central Colorado. For
almost 20 million years, this gaping crack in the Earth's crust has widened
and the San Luis Valley floor has slowly dropped while "hinged"
on the west to the San Juan Mountains. The sinking San Luis Valley has
created a catch basin for river gravels, eolian sands, volcanic ash-flow
tuffs and lake deposits and clays. About 15,000 feet of sedimentary fill
is now nestled below the Great Sand Dunes, all of which has been deposited
within the past 35 million years.
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