Engineer Mountain, a 12,968-foot peak rising just north of the Purgatory/Durango Mountain ski resort, is nestled in the San Juan Mountain Range, between the towns of Durango and Silverton, Colorado. Engineer Mountain shows off striking good looks, amazing geology, and incredible hiking access. A challenging 13 mile out-and-back trail from Coal Bank Hill off U.S. Highway 550 is an excellent way to see the mountain.
U.S. Highway 550 is one of the most stunning in the world, with this section designated as the San Juan Scenic and Historic Byway. Just north of Silverton is the “Million Dollar Highway.” The road from Durango to Silverton and on to Ouray is famous for it’s stunning scenery as well as it’s narrow lanes, plunging cliffside route, hairpin turns, and frequent closures due to avalanche and debris (so check the road for closures before driving!).
Geology:
Let’s talk about how the oddly striped mountain formed. There are two big things to keep in mind: how the rocks formed, and how the landscape (the canyons and shapes of the mountains) formed. Here, glacial ice and modern rain and snow carve the steep slopes. Long ago, ancient river systems deposited sandstones, siltstones, and conglomerates that brought sediment from eroding mountains. These sediments were intruded by magmas.
Ice
The entire Animas Valley was filled with approximately 2,000 feet of glacial ice during the Pleistocene ice ages. Engineer Mountain was one of the peaks tall enough to extend above the flows of ice. As glaciers moved, flowing downhill, the grinding ice wore away the bedrock, carving canyons. This icy erosion and the eventual melting of the glaciers exposed a striking band of cliffs that we see today, extending horizontally across the face of Engineer Mountain.
Magma
That dark cliff band is a what is called a “sill,” an igneous feature in which magma intrudes largely horizontally between sedimentary rock layers. The injection of these magmas pushed up overlying rock layers into a dome. These overlying rocks have been eroded from the top of Engineer Mountain, but their tilted remnants can be measured on the flanks of the nearby mountains.
What we see today
The Engineer Mountain sill is made of rhyolite with large potassium feldspar crystals. The magma that makes the sill intruded the Paleozoic sedimentary rocks between 17 and 14 million years ago. The vertical cracks in the sill formed during cooling of the magma. Known as columnar jointing, they form perpendicular to the cooling surfaces, in this case most apparent at the bottom of the sill. This vertical jointing is a common cooling feature, also famously seen in the Giants Causeway in Ireland and Devils Tower National Monument on Wyoming.
Geologic Maps: More than Color-by-Number
On a geologic map, different ages and types of rocks are different colors and patterns. The colorations that show where different rocks are are laid over a topographic map. This way, you can see both the terrain and the rock type. Additionally, geologic maps will show major geologic features, like faults.
Two maps below are geologic maps of the Engineer Mountain area. Here, the high ridge and peak of Engineer Mountain is shown in red hatched pattern labeled Tqt, “Tertiary quartz trachyte”; this coloring means the rocks are of Tertiary age (meaning in the last 66 million years). The blue-gray colors, labeled with the Cc indicates the rock is the Carboniferous Cutler Formation. This nomenclature has been modified as both mapping, dating of rock age, and timelines have become more refined over time.
In the screenshot below, the bright red blob that is Engineer Mountain is now labeled “Ti” standing for “Tertiary intrusive of an unknown age.” Since this map has been published in 1974, the actual age has been refined. More information on the ages of these intrusive rocks is here, published by Dave Gonzales at Fort Lewis College.
Your Turn
Have you hiked Engineer Mountain? Or other mountains in the San Juan Mountains? Do you have a favorite? Are there hikes on which you’re curious about the geology you see?
You can read more about the geology of Engineer Mountain and many other sites in “Colorado Rocks: A Guide to Geologic Sites in the Centennial State” available on this website and anywhere books are sold (Amazon, Barnes & Noble, etc.)