Recently my wife, Pam, and I were sitting on our front porch with our grandkids, Donovan (five) and Delaney (four), watching the light airplanes on their approach to land at the nearby airport.
During a lull in the air traffic, I lapsed into the chorus (with a slight change of lyrics) of an old George Strait song:
From my front porch you can see the sea…
Donovan immediately replied, “Papa, those are mountains, not the ocean!"
“Yes,” I said, “but way back in geologic time, before those mountains were even there, Colorado was covered with a giant sea.”
I knew Donovan and Delaney did not really care about the subject. They are only kids. However, as Rocky can attest, Colorado has indeed been covered with water a number of times throughout geologic history. To start with, Rocky was born in a vast, shallow sea, but that’s another story (see Geology Talk, Quarry, July 2008).
Seas invaded Colorado two more times during Rocky’s lifetime. Between Late Cambrian and Early Pennsylvanian time (510 to 300 million years ago), shallow tropical seas washed back and forth over Colorado (and much of the rest of the United States), depositing sediments that in Colorado accumulated to thicknesses of up to 3660m.
Some of these sediments ended up as limestone such as the Leadville Limestone, which was the host rock of the mother-lode of over $US82 million of silver mined in Colorado in the late 19th century. Other limestone has been used as a flux for making steel, a reactant to remove impurities from the product stream during sugar refining, and as plain old construction aggregate.
Fast forward to the mid-Cretaceous age (about 100 million years ago). Colorado was once again covered by a giant seaway (the Western Interior Seaway) that extended from the Arctic Ocean to the Gulf of Mexico. Before the sea retreated from Colorado, it left a thick blanket of sediments, which ultimately were transformed into a variety of rocks including sandstone for use as dimension stone, shale for brick-making and lightweight aggregate, and limestone for cement manufacture.
Here is some geologic information to impress the kids. During the mid-Jurassic (180 to 154 million years ago), the supercontinent Pangea began to split up due tithe forces of plate tectonics. Over geologic time, the associated rapid seafloor spreading opened up large new oceans that were underlain with thin, hot oceanic crust.
The new crust was less dense than the old, thick, cold oceanic crust that was overridden by the moving continents. The new, lighter crust floated higher on the mantle, and displaced seawater onto the land. That, combined with the fact that there were no polar ice caps to store water, is theorised as the cause of the Western Interior Seaway.
Cool, eh? When I pointed this out to the grandkids, Delaney giggled and said, “Papa, you’re funny.”