Today, the nearest beach is about12 hours from Montezuma County. But 100 million years ago, it was a different story, explained Mancos geologist Jon Callender at the Cortez Cultural Center on Mar. 4.
Callender’s presentation, “Geology and Resources of the Four Corners,” touched on how the area’s iconic rock formations came about.
The geological formations in Mancos and Cortez, including the sandstone cliffs of Mesa Verde National Park, were largely deposited between 100 to 75 million years ago, he explained, when the Western Interior Seaway covered the middle of the continent.
“Mancos and Cliffhouse are beach sands. ... In this area, we have some of the greatest evidence in the world of the in-and-out sea phenomenon,” said Callender. “That’s what we see in Cortez – we live on a beach.”
During this time period, the sea often advanced and retreated, depositing different types of sediment each time. About 100 million years ago, when the sea edge first reached the Montezuma County and the Mesa Verde area, it brought with it layers of beach-like sand. Millions of years later, this sediment was hardened and cemented into the Dakota Sandstone Formation, which today forms the base beneath the park and Montezuma Valley to the north, Calendar explained.
The Mancos shale, the gray rock that is visible at the base of the mesa in Montezuma Valley, was formed by the same phenomenon roughly 90 million years ago.
Another key point of Callender’s presentation was using present scientific occurrences to get a fuller picture of the patterns of the past.
“When you look at processes going on today and can see what went on in the past; the present is the key to the past,” said Callender.