Geology of Lake Mead
by Chris
I was excited on another level in that the Lake Mead basin has some really great geology. The story of how the earth changes over a billion and a half years is visible. The oldest rocks, mostly at the bottom of the basin and not visible date back 1.7 billion years, during the Pre Cambrian Era. Around 750 million years ago volcanos began to deposit material and erosion of the slowly uplifting land created sediments thousands of feet thick.
One of the most important geological features is called the great unconformity. The land uplifted more and was eroded over a long period of time so that the deposits that had been laid down during the next 250 million years are missing. You can see this as a sudden break in the layering angles of the rock as if someone sanded a piece of wood at an angle and then glued on another one with the grain at a different angle. Another 250 million years go by and the swampy seashore of this area and all the lands and seas suffer a mass extinction with 98% of all life suddenly dying out. Shortly afterwards (a couple of million years, just a blink in geologic time) the climate change from something like Florida to something like Saudi Arabia. You can see in the sand stones of this period, how the deposits were not all parallel (marine deposits) but instead, followed the changing slopes of the massive sand dunes. The deposits are visible as blood red mounds intermixed with other deposits visible in the photos of Granite Cove. Very cool.
A hundred million years ago everything from central Arizona over to the Appalachians and north way into the Arctic Ocean was a sea that expanded and contracted. New lifeforms developed like dinosaurs, other reptiles and birds and mammals, only to die out in the big meteor strike 65 million years ago. There is a detectable thin derbies deposit of this event visible in the basin. Camels, horses, and many other animals developed in North America. The current course of the Colorado only developed around 3.8 million years ago as the land was uplifted again. This caused the streams to run much faster and carry a larger load of debris that help cut the stone into the rugged shape today. At the end of the last ice age humans began to settle in the canyons of this area. You can still find ancient paintings and artifacts. Needless to say, I did my best to find some, but to no avail.