Colorado 14ers

The heart of the Colorado Rockies is astonishingly old. About 1.4 billion years ago, enormous volumes of magma welled up beneath what is now Colorado and cooled slowly into a series of giant granite batholiths; one of the biggest, the Longs Peak Batholith of Silver Plume Granite, today forms the summit block of Rocky Mountain National Park's highest peak (USGS). This granite intruded even older metamorphic schists and gneisses — a crystalline basement that would spend the next billion-plus years being buried, uplifted and stripped bare, twice over. The first range to stand here was not today's Rockies. In the Pennsylvanian Period, roughly 300 million years ago, the Ancestral Rocky Mountains rose across Colorado. Those mountains are gone — erosion erased them almost completely — but their debris survives. Coarse, feldspar-rich sand and gravel shed off the ancestral highlands hardened into the pink arkose of the Fountain Formation (USGS Geolex), the very rock that now leans against the Front Range as Boulder's Flatirons and the fins of Garden of the Gods. The Denver Basin preserves sedimentary records of both this event and the one that followed. The Rockies you hike today are the second act: the Laramide orogeny. Beginning roughly 70 million years ago, the oceanic Farallon plate began sliding beneath North America at an unusually shallow angle. Instead of building volcanoes at the coast, this 'flat slab' transmitted stress hundreds of kilometers inland, bowing up basement-cored block uplifts far from any plate edge. Between about 70 and 40 million years ago (NPS/USGS; some syntheses extend the event from 80 to 55 Ma), the ancient granite and gneiss were hoisted skyward, and the tilted sedimentary layers on the range flanks — including the Fountain Formation — were bent upright like books pushed over on a shelf. What came after the uplift matters just as much. Long erosion beveled the range into a gently rolling upland; the NPS notes that the smooth surfaces atop Trail Ridge and Flattop Mountain are remnants of that old erosion surface, now standing above 3,500 m. Then came the ice ages. Glaciers carved the cirques, knife ridges, U-shaped valleys and moraine-dammed meadows that give the park its alpine drama — young scenery chiseled into some of the oldest rock in the American West.