Geological journey

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— Driving south of Glenwood Springs on Colorado 82, motorists are greeted by imposing views of Mount Sopris. The conical twin peaks dominate the landscape from up and down the Roaring Fork Valley.

What isn't immediately apparent to travelers is the geology of the immense mountain. Sopris tops out at 12,953 feet, tall enough to hide views off taller peaks in the region like 14,130-foot Capitol Peak.

The second edition of a book that is familiar to many Coloradans, "Roadside Geology of Colorado." spells it out. Authors Halka Chronic and Felicie Williams explain that Sopris is a large "tertiary stock," an intrusion of crystalline igneous rock called quartz monzonite. The authors, who are mother and daughter, and hold advanced degrees in geology, point out the northernmost peak on Sopris exhibits a well developed rock glacier of large boulders. The rocks are actually creeping down the cirque below the summit in the same way that an icy glacier does in more northern latitudes.

The drive from Glenwood through Carbondale to Aspen reveals an amazing jumble of geology to people who know what to look for. The floor of the valley contains thick deposits of river gravel comprised of glacial outwash, there are red sandstone bluffs, terraces of Mancos shale and the obvious volcanic formations that give the little town of Basalt its name.

Chronic authored the first edition of Roadside Geology of Colorado in 1980. The original book is full of maps, and diagrams with black and white photographs of familiar landmarks. The second edition of the book updates half of the 136 photographs from the original edition and adds seven new roads, including Colorado 65, which crosses the Grand Mesa east of Grand Junction. Chronic divides the state into several regions and organizes the text around major highways. The book reveals the story behind the geologic sights that thousands of motorist cruise by each day without really appreciating the landscape.

For example anyone who has driven U.S. 40 between Steamboat Springs and Kremmling on the way to Denver, has admired the views of Wolford Mountain. Only those with a keen eye will recognize that a line of trees that knifes diagonally up the mountain is a tip-off to a thrust fault.

Chronic explains that the trees are able to grow on coarse Precambrian granite on the top half of the mountain. However, they fail to take root in the Cretaceous shales on Wolford's lower flanks.

The book explains that most of the visible geology in Colorado was shaped about 20,000 years ago, but there are rocks in the northwestern mountains that are 2.3 billion years old roughly half the age of the earth.

The book takes motorists on a geologic journey along U.S. 50 from the Kansas border to La Junta. It pierces the high San Juans and explains the three kinds of volcanic activity that are readily apparent on U.S. 550 between Silverton and Durango. West of Rifle , along Interstate 70, the book explains that formations like Mount Callahan demonstrate how the Colorado River has cut downward more than 3,000 feet over the last 10 million years.

The second edition of "Roadside Geology of Colorado deserves a place in the library of every Coloradan who is curious about the natural world. But it's far more useful if one keeps it in the glove compartment.

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