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The San Juan River boasts fascinating geology as it descends 84 miles from Bluff to Clay Hills. The river canyon exposes sedimentary rock layers chronicling 300 million years of Earth's history. From fossil-rich limestones to sandstones born of hardened dunes, each rock layer evidences a unique period of climate, geography and erosional and depositional processes. This layer-cake of Earth history--with younger layers deposited atop older layers--can be read from the bottom up like an open book of deep environmental history.
The oldest rocks along the river were deposited about 300 million years ago during the Pennsylvanian Period beneath what were shallow inland seas. The Earth's continents were unified and equatorial; bony fishes and amphibians were abundant and the first reptiles came onto the scene. The limestones deposited during this period--the Honaker Trail and Paradox Formations--contain numerous marine fossils such as brachiopods and chrinoids. Alternating layers of shale between the limestones evidence rising and falling sea levels resulting from alternating ice ages and interglacials.
In time inland seas gave way to bays, estuaries and freshwater floodplains. Layers deposited during the Permain Period (about 290 to 250 million years ago) evidence this transition from a marine to terrestrial environment. The terrestrial "red beds" such as the Halgaito Formation and the Cedar Mesa Sandstone contrast sharply with the gray limestones and shales top which they sit. As these layers were being deposited rreptiles, ginkgos and conifers became abundant while trilobites and placoderms went extinct.
Atop the Permian rocks are Triassic and Jurassic rocks, alternating cross-bedded sandstones and red shales reflecting broad shifts between vast arid deserts and more mesic climates. During this "age of dinosaurs" 250 - 150 million years ago, the unified continent of Pangea separated, skates, rays and bony fishes dominated the seas and the first lizards and the mammals came onto the scene. The Chinle formation, for example, contains dinosaur fossils, petrified wood, and trace amounts of Uranium.
Today's landscape surrounding the San Juan River was further shaped by tectonic and erosional forces. The river cuts through a massive upward fold in the Earth's crust, called the "Monument Upwarp," the ripple-effect of a massive collision between the Pacific and North American plates 70 million years ago. As upthrust land eroded, stark geographic features resulted from the relatively hard rock layers. Examples include the Comb Ridge monocline and Clay Hills at the east and west edges of the Upwarp, respectively.
The river canyon itself is thought to have resulted from down cutting that occurred as the river--once-meandering through a soft sandy plain--began incising into the sedimentary layers below. First flowing from highlands to the southwest, uplift to the northeast is thought to have switched the river's direction as it drained massive inland lakes some 10 or 20 million years ago. Once entrenched, the canyon continued down cutting through the alternating glacials and interglacials that have proceeded until today.