Current Earth Science

Humans, the Short End of the Time Scale

The long-winded history of the Great Plains is recorded in the geological features we see today. The Precambrian shows itself in outcrops of the pink Town Mountain granite, the Packsaddle schist and Valley Springs gneiss. These rocks tell a story of massive chambers of boiling-hot magma being pushed to the surface to cool, and surrounding rocks being metamorphosed by intense heat and pressure. In the Paleozoic and later in the Cretaceous, ancient shallow seas covered the inner continent and produced carbon-rich rocks. Limestones, shales, and fossilized reefs are scattered throughout the Great Plains and nearby areas, evidence that the area was not always the ‘Great American Desert’. Decaying plant matter built up and compressed over time resulted in coveted coal deposits. When the waters retreated, the temperatures started going down, too.

Now, the Great Plains are dusted by a layer of younger sediments—sand and gravel and soil—eroded from the Rocky Mountains on the western border. These sediments help trap water underground in aquifers, supplying the region and its inhabitants. It is no surprise that pockets of water have been attracting all sorts of living beings for thousands of years, and early humans were no exception.

The common consensus among archaeologists is that humans began living on the Great Plains at least 11,000-11,500 years ago, based on carbon dating of various artifacts like bones, stone flakes and tools. This time overlaps with the last Ice Age, and the time of giant creatures like mammoths and saber-tooth cats (see Miocene and Mammoths). In 2005, a study took advantage of some broken mammoth bones that had been on display at the Denver Museum of Nature and Science for nearly 30 years. The bones appeared to have been broken by humans rather than by natural means, and were carbon dated at about 12,200 years old. Many scientists are drawn to the significance of discovering the oldest site with clear evidence of human habitation, and so research continues.

Humans have been fascinated by the challenges and opportunities offered by life on the Great Plains for a very long time, whether because of water and food resources, the hunt for gold and precious materials, agriculture, or an adventure. This curiosity is prominently realized in the mid-1800s, when Americans were surveying the Territories and some people thought that rain would follow the plow.

 

Featured photo courtesy of U.S. National Archive:

Hikers Terry Mcgaw and Glen Denny Enjoy a View of the Maze from the Top of a Ridge Separating Two Canyons. The Maze, a Wild and Rugged Region in the Heart of the Canyonlands, Has No Footpaths, and Hikers Enter by Means of Ropes Or Steps Cut in the Rocks,

 

See the following sources for more info:

http://geology.teacherfriendlyguide.org/index.php/rocks-sc/rocks-region4-sc

https://www.geosociety.org/documents/gsa/timescale/timescl.pdf

Denver Post Staff Writer. “Museum’s Mammoth Bones Exhibit New Life After Nearly 30 Years on Display, Fossils Found in Kansas Are Causing Scientists to Re-examine When Humans Arrived on the Great Plains. (DENVER AND WEST).” The Denver Post (Denver, CO), 2005.

Deep Time Snapshot

The Cretaceous Interior Seaway

Jurassic Park helped to give dinosaurs their fame, but this really wasn’t when dinosaurs ruled the Earth. The Cretaceous period, starting about 145 million years ago, was the real heyday of our giant mesothermic friends (most dinosaurs were not ectothermic, or cold-blooded!). Until their extinction around 66 million years ago, dinosaurs were spread across the globe, including in the Great Plains region. At this point, Pangea had broken apart and North America was starting to look like the massive continent we know today.

Even more interesting, the North American landmass was home to a Cretaceous inland sea. The Western Interior Seaway was a result of incredibly high sea level, where a shallow ocean covered a massive part of the continent. “At its maximum extent, the seaway extended for 4,800 km from the North Slope of Alaska to northern Mexico and was approximately 1,620 km wide from central Utah to Minnesota,” according to a U.S. Geological Survey report. This includes much of the Great Plains region.

Evidence of this inland sea lies in the paleontology—the fossils—of the region. Geologists have found an incredibly diverse variety of marine fossils all over the landlocked states, even here in Arizona. The fossils found are not just shallow water creatures, though some extensive coral reef remains have been studied. Giant turtles, ammonites, crinoids, sharks and fishes have been discovered in areas that are now deserts and grasslands. On top of that, giant marine beasts resembling the Loch Ness monster, like Mosasaurs and Plesiosaurs who needed large expanses of salty ocean water to survive, have been found. Right along side these are the bones and tracks of dinosaurs who tromped on the muddy shores.

The rock that hosts the marine fossils also tells the tale of deep ocean covering North America. Chalk, limestone and dolostone are carbonate rocks, created when microscopic sea-critters died and accumulated on the ocean floor, eventually compacting into solid rocks.

By the end of the Cretaceous, the Rocky Mountains were beginning to form, and the Western Interior Seaway was retreating. The dropping sea level and receding inland sea led way to swampy environments, and later, the formation of coal.

 

Featured photo courtesy of U.S. National Archives:

Aerial View Upstream of Boom Site in Monument Valley, Utah Where Oil Spill Into the San Juan River Was Contained before Flooding Caused Overflow of Oil and Debris Into Lake Powell, 10/1972

 

See the following sources for more info:

http://geology.teacherfriendlyguide.org/index.php/fossils-sc/fossils-region4-sc

https://www.geosociety.org/documents/gsa/timescale/timescl.pdf

Roberts, Laura N. Robinson, Laura N. Robinson. Kirschbaum, Kirschbaum, Mark A., and Geological Survey, Issuing Body. Paleogeography of the Late Cretaceous of the Western Interior of Middle North America: Coal Distribution and Sediment Accumulation. U.S. Geological Survey Professional Paper; 1561. Washington: Denver, CO: U.S. G.P.O.; U.S. Geological Survey, Information Services, 1995.