Uncompahgre Mountain Range, which bound the San Juan Basin on the northeast. Mesozoic Subduction During the late Jurassic, continental collision of the Farallon and North American plates resulted in low-angle ("flat slab") subduction beneath the western margin of the United States. Pressure on the underlying lithosphere resulting in a "depression" of the continents interior, and this allowed for the formation of the Inner Cretaceous Seaway ( Western Interior Seaway). This began the transition from terrestrial sedimentation during the late Paleozoic and early Mesozoic to a shallow marine basin, as waters from the Arctic and Gulf regions poured into the center of the continent. Cenozoic Evolution During late Cretaceous to early Tertiary, compressional forces (continued subduction of the Farallon Plate) continued to act and caused the uplift of the modern Rocky Mountains via the Laramide Orogeny. Early-Tertiary tilting towards the northwest resulted in over of erosion in the southeast. As compression shifted to extension and the formation of the Rio Grande Rift began, volcanism dominated the area throughout much of the Eocene and Oligocene. Uplift in the northwest and continued deposition brought the basin to its current-day configuration. Components The San Juan Basin is an asymmetrical syncline with three components: the Central Basin Platform, the Four Corners Platform, and the Chaco Slope (a.k.a. the Chaco Homocline). The basin is bound on the northwest by the Hogback Monocline (separating the Central Basin and Four Corners platforms), on the northeast by the Archuleta Anticlinorium, on the east by the Nacimiento Uplift, and on the south by the Zuni Uplift. Depositional History Paleozoic Prior to collision, Mississippian- and Pennsylvanian-aged units were deposited during various marine environments, e.g. the Leadville Limestone and the Pinkerton Trail Formations. Once the supercontinents collided (see Tectonic Evolution above), the subsidence of the Paradox Basin and the uplift of the Uncompahgre highlands allowed for enormous amounts of sediment to shed of the highlands via Permian fluvial systems. The Rico Formation represents the transition from Pennsylvanian marine deposits to Permian terrestrial deposits of the Cutler Formation. The Permian continued to be a time of terrestrial deposits, including late-Permian eolian deposits. Mesozoic The Cretaceous was a time of three major transgressive-regressive cycles, as eustatic changes in sea-level caused fluctuations on the Western Interior Seaway's shoreline. The San Juan Basin was conveniently located in the western margin of the seaway and recorded these cycles in the stratigraphy (see Stratigraphy below). The westernmost extent