CHIMNEY ROCK
-- GEOLOGY --
Long before humans found the mesas and spires of Chimney Rock, this impressive landmark was created by the forces of nature: millions of years of slow settling of mud in a shallow sea; gradual drying of the sea and migration of beaches and rivers across the ancient basin; thick, humid swamps dominated by dinosaurs and giant insects; the catastrophic birth of mighty volcanoes and their scouring, deadly eruptions; slow but constant uplift and tilting of the land; millenia of glaciers and vast floods as the glaciers melted away; erosion of the exposed beach sands and ocean-bottom muds (now solidly cemented into rock); and - finally - the arrival of the first Americans to marvel at this incredible work of the Earth. (Geologic Timeline: Today-35 Million Years Ago, 40-75 Million Years Ago)
Chimney Rock's twin spires are entirely natural in origin, the erosional remnant of a thick sequence of sedimentary rock laid down in the late Cretaceous Period, from about 100-70 million years ago. The oldest rocks exposed in the area (to the north of Chimney Rock) are the terrestrial and marine-shoreline sandstones and siltstones of the Mesa Verde Group; above these beds lies the shallow sea-bottom Lewis Shale, which forms the slopes and canyons of the Chimney Rock area. The shales are capped by the tidal, beachfront, and river sand layers of the Pictured Cliffs Sandstone. It is this unit that forms the mesa tops and dramatic stone pillars of the Chimney Rock formation. Erosion has removed the rock units which once lay above the Pictured Cliffs Sandstone, but these rock layers are exposed to the south of Highway 151 and into northern New Mexico.
Chimney Rock beaches and floodplains were millions of years in the past when the dinosaurs were extinguished, 65 million years ago, but the rock layer that records that disaster lies not more than a few hundred feet above Chimney Rock's highest point, at least stratigraghically (as the rock layers originally were deposited.) In general, sedimentary rocks are deposited in relatively horizontal layers, so the numerous rock beds that make up Chimney Rock country were originally flat. But deposition is not the only force in geology that shapes the land. As the North American continent broke apart from the supercontinent of Gondwanaland and drifted west on its tectonic plate, it began to stretch and buckle. Locally, the great depression called the San Juan Basin formed in northern New Mexico and today's Rocky Mountains began to rise, a time known as the Larimide Orogeny. The land between these two geologic provinces lifted and tilted to the south. Its northern edge fractured, folded, and crumpled, cut by faulting and breached by erosion. These rock layers are exposed as 45-degree tilted beds along the north side of US Highway 160, west of the Piedra River. Further south, though, the rocks lay in a smooth slope of about 7 degrees, dipping into the northern edge of the San Juan Basin.
Erosion proceeded to remove the rock layers as they were uplifted. The rising mountains to the north and other worldwide climate factors altered weather patterns. From the early Pleistocene Epoch (about 2 million years ago), glacial periods also affected the climate. It is likely that the Pictured Cliffs Sandstone was exposed as a narrow ridge during the latest Pleistocene or earliest Holocene, about 100,000 years ago, probably as a series of spires and walls and possibly arches. After the most recent retreat of the Wisconsin Glaciation (about 15,000 years ago), the most active erosion ended, leaving the twin formation we call Chimney Rock, standing free of the softer shales and weaker sandstones that once surrounded and covered it.
The appearance of the twin pillars high above the river, named in 1776 by Dominguez and Escalante as El Rio de la Piedra Parada - the River of the Standing Rock, has changed little since that time, but in the history of the Earth, a single millenium or two is nothing. Barring an earthquake, the Chimney Rock pinnacles should easily endure another fifty or hundred thousand years. Nature will eventually triumph, however, and sweep them away, grain by grain, to the ocean floor - the ultimate doom of all rock.
*This brief introduction was excerpted from "Geology of Chimney Rock", compiled by Glenn Raby, USDA Forest Service, Pagosa Ranger District, San Juan Public Lands.
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Hours of Operation: In-Season May 15 - September 30, Daily 9:00 a.m. - 4:30 p.m.
Guided Walking Tour Schedule: 9:30 a.m., 10:30 a.m., 1:00 p.m., 2:00 p.m.
Location: 3 miles South of Hwy 160 on Hwy 151 (map)
Mailing Address: Chimney Rock Interpretive Program, P.O. Box 1662, Pagosa Springs, CO 81147
Phone: (970)883-5359 Visitor's Cabin In-season, (970)264-2287 Leave Message Off-season
E-mail: chimneyrock@chimneyrockco.org
Download Chimney Rock Brochure
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