The Colorful Folds of the Sierra Nacimiento Mountains

Sedimentary rocks hold many charms for geologists. They contain the Earth’s archives, recording the distribution of ancient environments, outlining areas of subsidence and uplift, and tracking changes in climate and fluctuations in sea level over the ages. They preserve the history of life on Earth in the fossil record. They contain mineral fuels like coal and petroleum.

But there is an additional, almost incidental aspect of their record-keeping abilities which never fails to fascinate even casual observers. It’s tucked into the formidable phrase “original horizontality”.

Because sediments at the Earth’s surface are originally laid down in approximately horizontal layers, dispersed by moving water or blowing wind and settling under gravity, we can infer subsequent deformation of the outer crust by their displacements from the horizontal. Deformation by flexure – or folding – is one of the most striking manifestations, and some truly spectacular examples can be found here in New Mexico along the southern flanks of the Sierra Nacimiento Mountains, only a short drive west of Albuquerque.

Here is a Google Earth Image of the San Ysidro anticline, an up-arched buckle in the Earth’s crust beautifully outlined by a layer of white gypsum encircling a core of red shale:

The San Ysidro Anticline

The San Ysidro Anticline

Erosion has scraped out the soft center of this fold, exaggerating its appearance. Other sedimentary beds tilt away from the elongated center of the fold – its axis – in all directions. The axis of this anticline dips below the surface – or plunges – toward the lower left.

This part of New Mexico is just on the edge of the Colorado Plateau, which stretches from here far to the north and west. The Colorado Plateau is famous for its colorful strata, laid out in buttes and mesas or exposed in deep, sheer-walled canyons, and these wild color contrasts make this area of folding more spectacular than most.

Here is a view of the fold taken from the opposite direction which includes its companion fold, a plunging syncline:

Complementary plunging syncline and anticline

Complementary plunging syncline and anticline

Synclines are down-buckles in the Earth’s crust, U-shaped in cross-section. Rumple your napkin up by pushing it across the table and you’ll get the idea of why these folds come in pairs.

Sedimentary rocks make a cover – a sort of thin blanket – at the top of the Earth’s crust, resting on older and unrelated crystalline rocks like granite or schist. Geologists often refer to this ancient foundation as the crystalline basement. Just to the north of these folds, an upthrust of the crystalline basement rocks in the Sierra Nacimiento has flexed these same strata sharply upward, almost to the vertical:

Along the western front of the Sierra Nacimiento

Along the western front of the Sierra Nacimiento

Scenes like this are common throughout the Southern Rocky Mountains and the Colorado Plateau. Some famous examples include the Flatirons near Boulder, Colorado and the Garden of the Gods, near Colorado Springs.

One pleasant byproduct of folding sedimentary strata, at least for geologists, is the fact that you can go up and down the geologic record by walking among the tilted strata, rather than scaling a cliff or drilling a well. Here is the transition from the colorful shales and sandstones of the Morrison Formation, to the right, into the duller grey and yellow shales of the Dakota Formation, off toward the left:

Tilted Mesozoic strata in the east limb of the San Ysidro Anticline

Tilted Mesozoic strata in the east limb of the San Ysidro Anticline

A scramble along one of the dry washes that cuts the flank of the anticline will let you examine a huge thickness of rock.

Of course, you may decide that today is not the day for scrambling down a gulch of slippery shale, and are content to simply enjoy the view:

Enjoying New Mexico

Enjoying New Mexico

It’s all good.

One thought on “The Colorful Folds of the Sierra Nacimiento Mountains

  1. Judith Renbarger

    I really enjoyed the illustrations and the wonderful, informative dialogue which went with them.

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