of the seaway (a.k.a. maximum transgression) was recorded by the Lewis Shale, which eventually graded into the Pictured Cliffs and the Fruitland Formation as the shoreline made its final retreat. Cenozoic The Western Interior Seaway's shoreline regression resulted in ample swamps, lakes, and flood plains; this resulted in the coal-rich formations of the late Mesozoic/early Cenozoic (e.g. Fruitland Formation and the Kirtland Shale). Eocene/Oligocene volcanism resulted in large volcanic aprons that covered thousands of square-kilometers, and these volcanic fields sourced the Cenozoic units the Ojo Alamo (sourced from the west), and the Animas and Nacimiento formations (sourced from the northeast). Uplift in the northwest (and subsequent erosion), and continued deposition (e.g. the San Jose Formation) brought the basin to its current-day configuration. Stratigraphy Precambrian Little is known about Precambrian units due to poor outcrop exposure and poor well-control. Precambrian rocks consist of quartzite, schist, and granite, and rocks are overlain unconformably by younger Paleozoic units. Paleozoic Little is known about the stratigraphy of the Paleozoic. Of the >40,000 wells drilled in the San Juan Basin, only about 12 have penetrated deep enough to come in contact with Paleozoic units. Additionally, poor outcrop exposure and lateral facies changes complicate the defining and correlation of these units. Devonian The Ignacio Formation consists of quartzite, sandstone, and shale layers. The unit is argued to be Late Devonian in age and was deposited by an east-trending transgression (unconformably) covering Precambrian rock throughout the present-day Four Corners Platform. Preservation of the Ignacio is poor and mostly limited to localized areas of down-thrown faults. The Aneth Formation consists of dark limestone, clay-rich dolomite, and black shale or siltstone layers. This Late Devonian-aged unit has a depositional environment similar to that of the Ignacio Formation, though the two formations rest unconformably over one another. The Aneth Formation is found only in the subsurface, cropping out nowhere. The Elbert Formation consists of two members: The McCracken Sandstone Member consists of poorly-sorted sandstones sourced from the east. Exposures can be found in the San Juan Mountains unconformably overlying Cambrian units. The unnamed upper member consists of green shales, white sandstones, and thin limestone or dolomite beds deposited in a tidal-flat environment. The Ouray Formation consists of fossil-rich (brachiopods, gastropods, crinoids, etc.) limestone or dolomite beds that conformably overlie the previous unit. Fossil faunas are indicative of a Late Devonian (and possibly early Mississippian) marine environment. Mississippian The Leadville Limestone