The shale that the world knows as Mancos

At least a few of you know that the Mancos Shale, which ranges throughout much of Colorado, parts of Utah, Wyoming and Montana and was even deposited in parts of Canada, was named after the town of Mancos.

In 1899, a geologist by the name of Cross ventured into southwestern Colorado and found the town of Mancos sitting on a layer of shale. He named that formation the Mancos Shale.

Here in the valley, the Mancos Shale is 2,300 feet thick and was formed about 99 to 90 million years ago in a near shore area of a large inland sea.

The fossils in this formation are principally ammonites, clams, oysters, scallops, and snails. I remember hunting for shark teeth in a particular layer of the Mancos Shale when I was young, just like other people would hunt for arrowheads.

The potentially toxic elements in the Mancos Shale are arsenic, selenium, chromium and mercury.

The Mesa Verde Sandstone, which lies on top of the Mancos shale, was deposited 90 to 75 million years ago. The lowest layer of that sandstone has been named the Point Lookout Sandstone. It was deposited as the inland sea retreated. Only a few scattered fossils have been found in this layer.

Directly above the Point Lookout is the Menefee Formation. It was formed as the inland sea completely withdrew, leaving behind a flat coastal plain. Swamps developed as small, slow-moving streams wound their way to the sea in northeast Colorado. As plant material decayed, coal seams began to form. Leaf and other fossilized plant remains have been discovered in this layer.

The coal that was mined in the Menefee foothills during the first third of the 20th century came from the Menefee layer of the Mesa Verde Sandstone.

Many millions of years later, Hesperus and other peaks in the La Plata Mountains were thrust up, and erosion gradually wore away the Mesa Verde Sandstone layers that had covered the Mancos Valley.

Ancient Ones moved through our general area some 12,000 years ago and left Clovis points and other such chipped tools.

And that's the most ancient history of the Mancos Valley I have been able to come up with.

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