The mesa is capped with basalt lava flows that are almost 11 million years old (radiometrically dated at 10.76 Ma). Beneath this caprock, lies unmistakable Colorado River pebbles and cobbles that are about 10 to 20 feet thick. This suggests that the fluid lava likely flowed in and was confined in a valley of the ancestral Colorado River. Since 11 million years ago, the river has dissected a new course to the north of Grand Mesa and this lies a little less than 5,000 below the old gravels. This gives an average incision rate for the Colorado River in the last 11 million years of about 460 feet every 1 million years. Of course, averages in the Southwest are sometimes meaningless, since climate and tectonic uplift rates may vary throughout geologic time. But it gives a sense of how fast canyons have been carved on the Colorado Plateau.
In July, I took a nine day trip through Colorado to see some new geologic sights with my colleague Jack Share. Here are some pictures and I will be posting more Colorado geology stories in the days to come.
The valley definitely looks underfit for the amount of water in it today. These grasses filled the floor of the valley and we watched dragonflies dance in this green paradise.
Below are a few pictures of Miocene age animals that might have been roaming around when the Grand Mesa lava flow was emplaced 11 Ma.