Conglomerates! – September 23, 2020


Yesterday Ricardo and I climbed Challenger Point (14,081 ft.), a 12.5 mile round-trip hike with 5400 vertical feet of gain. As a geologist, I always love to look at the rocks that we’re climbing and this peak was no exception! The parent mountain range of Challenger is the Sangre de Cristo of Colorado and it is home to some unique and beautiful outcroppings of the “Crestone Conglomerate.”
A conglomerate is a sedimentary rock that is composed of a substantial fraction of pieces (“clasts”) of other rocks ranging in size from pebbles to boulders, held together by natural “cement.” In the case of the Crestone conglomerate, this includes a mix of Precambrian-aged sedimentary and metamorphic rocks like sandstones, quartzite and schists, held together by a matrix of silica cement (aka, silicon dioxide, SiO2, the building block of many rocks). The Precambrian time period is the earliest part of Earth’s history, dating from 4.6 billion – 541 million years ago.
So, how did this conglomerate form? 300 million years ago, before the modern Rocky Mountains were uplifted (70 – 40 million years ago), the Ancestral Rockies dominated the landscape. As uplift ceased and erosion began to take over, massive amounts of rock were shed from the mountains. You may notice that the clasts are very rounded in shape (versus angular) and this is a clue that they were tumbled for some distance by running water, smoothing out their sides. As these clasts made their way down the mountains via rivers and streams, they eventually were deposited in flat-lying basins below, piling up thousands of feet of mixed sediments. The pressure of this pile-up and the flowing of groundwater through it, carrying dissolved elements in it (which eventually precipitated as a cement between the clasts), turned the mixture into the conglomerate we see today, brought up to the surface about 30 million years ago when the San Luis Valley segment of the famous Rio Grande Rift (which stretches from Colorado to Mexico!) began opening up.