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The Mancos Shale: Mud, Mud, Glorious Mud!

An outcropping of Mancos Shale in the Cerrillos Hills

An outcropping of Mancos Shale in the Cerrillos Hills

Of all the rocks in the American West, perhaps the least loved – or at least, the most overlooked – is the Mancos Shale. Rather than forming the spectacular buttes and spires that draw people to the Southwest, this dull grey rock underlies the long hot drives between the National Parks and Monuments you are longing to visit. It forms sullen badlands, here and there, that seem to suck up the famous light rather than casting it back to you in glowing cliffs and purple-shadowed mountains. Even the toughest desert grasses struggle to grow on it. And yet, as so many other unpromising encounters in life do, these rocks conceal a fascinating story behind their reclusive and crumbling facade.

First, a word about a name. In order to reconstruct the history of the Earth from the rocks – or for that matter, to plan a mining venture or prepare a suitable foundation for a big construction project – different kinds of rocks are mapped and cross-sectioned on a variety of scales. To this end, packages of similar rocks are assigned to what are called formations. To a geologist, a formation is not an interestingly shaped rock, like, for instance “Camel Rock” just north of Santa Fe, or the “Sword of Damocles” in Carlsbad Caverns, but rather, a package of rock that is easy to distinguish from surrounding rocks,  and sufficiently large enough to be plotted and traced out on a map. Formations are given formal names, with capital letters to prove it, based on a geographical feature near the place where the unit is particularly well-displayed, or first described. It so happens that the thick grey beds of the Mancos Shale crop out beautifully near the town of Mancos, Colorado, close to Mesa Verde National Park. Hence the name.

Shale is the most abundant of the sedimentary rocks because it is made of that most ubiquitous of substances: mud. It doesn’t have any of the seductiveness of sandstone, hinting of ancient beaches and desert dunes, nor the allure of limestone, redolent of tropical seas and atolls. . . but there sure is a lot of mudstone in the geologic record. Shale is mudstone that exhibits, well, shaliness: 

Shale

Shale

It’s fissile – the shale splits easily into thin plates and flakes, in a crude reflection of the billions of microscopic platy clay minerals, aligned by settling out of water – picture throwing a deck of cards up in the air and noting how they end up flat all over the floor – along with the effects of tiny partings of coarser silt among the mud. This property is particularly well developed when the mud has settled in quiet water – for instance, deep on the floor of the ocean, below the stirring effects of waves. Shale comes in many dull earthy colors, reflecting its muddy heritage, but the Mancos Shale is generally a dark grey to nearly black, thanks to its rich content of organic carbon. That’s another clue, by the way, that this mud collected in somewhat stagnant waters, where  decay was inhibited by low oxygen.

Evidence of life isn’t entirely missing from these ancient muds, however, and that’s the reason for this post. I know there are a fair number of fossil lovers out there, and if you have a walk in our new Cerrillos Hills State Park, south of Santa Fe, or brave the ruts of the old Waldo Road from Cerrillos to I-25, you can spend some pleasant time looking for marine fossils from the seas of the Cretaceous Period. Two important caveats: First – no collecting is allowed in the State Park. You can look, photograph, or sketch, but the fossils stay. And second: if you decide to try the Waldo Road, be sure you have a moderately high clearance vehicle, and don’t even think of taking the road if rain threatens. The Mancos Shale is full of “swelling” clays and the dusty roadbed will liquify in minutes into a dangerously slick and tenaciously sticky mess, quite capable of fossilizing your car. No one will come out to help you, either, until the mud dries.

The "Devil's Throne" along the Waldo Road

The "Devil's Throne" along the Waldo Road

 In places the bedding planes of the shale are covered in what are called trace fossils. Trace fossils are records of animal activity preserved in sediment, rather than remains of the animal itself. A footprint is a good example. But check these patterns out:

Feeding traces in the Mancos Shale

Feeding traces in the Mancos Shale

Some sorts of creatures spent their time browsing around on the muddy floor of the sea, either searching for tidbits to eat, or, more likely, eating the mud itself to strain out the organic matter in it, for nutrition. Feeding traces like these, as well as filled-in burrows, and the occasional tiny rows of tracks (looking like someone was trying to row a boat in mud) are perfect examples of trace fossils, and are very characteristic of the Mancos Shale. At times the sea floor was literally crawling with life, in spite of the murk and low oxygen.

With a little more luck you’ll find some of these:

Inoceramus fossil

Inoceramus fossil

This is the impression of a marine bivalve, called Inoceramus, which was common worldwide in the Cretaceous seas that covered much of the planet.

With even more luck – as in more luck than I’ve had so far – you might find an ammonite fossil, or a shark’s tooth that drifted down long ago to bury itself in the dark mud at the bottom of the Interior Cretaceous Seaway.

It’s hard to imagine, as you stroll through these arid and dusty hills, that all this country, from horizon to horizon, and from Gulf of Mexico to Arctic pole, was covered by a shallow ocean, less than 100 million years ago. And if you are inclined to stimulate your imagination even further, find a copy of the amazing new book “Ancient Landscapes of the Colorado Plateau“, by Ron Blakey and Wayne Ranney, published by the Grand Canyon Association. It’s the closest thing you’ll find, currently, to sneaking a peak at God’s old atlas of the Earth.

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