Some rock history you (probably) didn’t know

A storm passes over Mount Sopris on the afternoon of July 31.
Christopher Mullen / Post Independent |

Mount Sopris isn’t a volcano. The molten rock that formed the granite core of the lower Roaring Fork Valley’s most prominent peak cooled 10,000 feet underground. If any found its way to the surface, no evidence remains.

Still, the myth that Sopris gets its twin summit from a Mount St. Helens-style eruption is so pervasive that Colorado Mountain College devoted one of several geological information signs at the Glenwood Caverns Adventure Park to debunking it. The panels, organized by Beth Zukowski, designed by Alice Sjoberg, and commissioned by park owners Steve and Jeannie Beckley, were installed in June and also include information on Glenwood Canyon, the Grand Hogback and the general geological history of the area.

For those without the time or energy to make the trip to the upper deck of the Lookout Grille to see the signs, the Post Independent sat down with CMC professor emeritus Garry Zabel, who provided most of the signs’ content. Zabel is the first to admit that geologists don’t have all the answers, but they can infer a great deal about how the area came to look the way it does.

Let’s begin at the beginning.

The oldest rocks in the Roaring Fork Valley are mostly granite and gneiss dating back at least 1.7 billion years to the Precambrian, when most life was too small and simple to be seen with the naked eye. Many of the peaks along the Continental Divide are made of the stuff, but in Garfield County it’s mostly seen as the very lowest layer of rock in Glenwood Canyon and some of the deep drainages coming out of the Flat Tops.

The next oldest surviving rocks are several types of sedimentary rocks including beach sandstones and limestone, which formed as microscopic marine organisms settled at the bottom of an inland ocean on a continental shelf around 350 million years ago. It is made of calcium carbonate, which dissolves easily by ground water and produces features like Glenwood Caverns and Hanging Lake. Under certain conditions, limestone also forms marble — but we’ll get to that later.

Around 300 million years ago, a pair of mountain ranges collectively called the Ancestral Rockies formed on either side of the Roaring Fork Valley. The sea dried up, leaving a thick layer called the Eagle Valley Evaporite. The pale gypsum is visible throughout the area, but is most apparent near the town that takes its name. There, it is mined to create plaster and drywall that lines the internal walls of most buildings.

Streams and rivers coming off these ancient mountains deposited sandstone, conglomerates and shales, which on this side of the Rockies is called the Maroon Formation. This brittle stone is what makes the Maroon Bells particularly treacherous to climbers compared with the sturdy granite peaks to the east and west.

Moving forward, 150 million years ago finds us in the middle of the age of the dinosaurs. The ebb and flow of another inland sea laid down several new formations. The Morrison Formation, made of sand and mud along the coast of another inland sea and the source of most Western Slope dinosaur fossils, lies under Dakota Sandstone, a seashore beach deposit. Mancos Shale, which holds most of the area’s natural gas, and the Mesaverde Formation, which hosts the area’s coal, both date from near the end of this era.

The modern Rocky Mountains began to take shape in a period of uplift called the Laramide Orogeny, which began around 70 million years ago during the twilight of the dinosaurs and continued until about 40 million years ago.

The mostly vertical layers of the Grand Hogback, a narrow ridge that runs from McClure Pass to Meeker, mark the official boundary of the Rocky Mountains. To the west, the faulting and folding of mountain-building become less prominent, with most layers lying more or less horizontal. To the east, the scene is more chaotic.

Just over 30 million years ago, massive bubbles of molten rock — called plutons — formed between the present-day locations of Carbondale, Crested Butte and Crawford. After they cooled, erosion uncovered them, producing many of the West Elks’ most prominent peaks, including Mount Sopris. The pluton that would become Treasure Mountain also heated, but didn’t melt, some of the adjacent limestone, producing the beautiful metamorphic Yule Marble which was used in the Lincoln Memorial and the Tomb of the Unknowns and in many other buildings and monuments.

Although Sopris is not a volcano, Basalt Mountain is. Large parts of Missouri Heights northeast of Carbondale are capped by 10 million-year-old layers of basalt from the mountain. There’s other evidence of volcanic activity in the area, including more basalt and Dotsero Crater.

Some of the valley’s most significant features are comparatively new.

The Colorado River formed as early as 10 million years ago, with the bulk of Glenwood Canyon carved in the last 3 million to 10 million years.

The Rocky Mountains are geologically young, which makes for a geothermal gradient that is higher than normal, meaning that ground water doesn’t have to go very deep to be heated by the earth’s internal temperature. It then makes its way back to the surface at regional low points, often along the river. The hot springs that feed the Glenwood Hot Springs pool are the most notable of these, but they are common throughout the valley and the Rockies.

Although geology tells us a great deal about the past and has many present applications in construction, engineering and extractive industry, any extrapolation into the future is tenuous at best.

Right now, Zabel says, erosion is winning in the Rocky Mountains. Millions of years from now, when the Pacific Ocean disappears and North America collides with Asia, western Colorado could be nearly flat again, or partially under water as it has been off and on for the last half billion years.

If so, like the ancestral Rockies, the presence of the modern Rockies will be only inferred by any future geologists, and there’s unlikely to be much evidence of our time here.

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