Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 124. The San Juan River has carved majestic canyons through a thick sequence of marine sedimentary rocks of Pennsylvanian (Upper Carboniferous) age in southeastern Utah. The superimposed canyons were cut across the huge Monument Upwarp in late Tertiary to Recent times, exposing a natural cross section of the cyclic strata, including algal bioherms, in unexcelled magnificence. Deepest and oldest rocks exposed are evaporites and carbonates of the Paradox Formation (Middle Pennsylvanian) along the southwestern shelf of the Paradox evaporite basin. The Paradox is a pull-apart basin that sagged into existence during the Ancestral Rockies orogeny, the American counterpart of the Hercynian orogeny of Eurasia. Upper canyon walls are composed of carbonate and clastic cycles of the Late Pennsylvanian Honaker Trail Formation.