fossiliferous Silurian and Devonian strata occur preserved as the fills of solution and collapse structures that vary in size from large structural sinks to crack fillings a few inches or less in width. Isolated deposits of fossiliferous Starcke Limestone preserved in ancient sinkholes developed in the Ellenberger Group provide definite evidence of the Llano region having been inundated by marine waters at least once during the Silurian Period. Fossiliferous Devonian limestones of various types that are preserved in cave fills, collapse depressions, and other paleokarst features develop in the Ellenberger Group also demonstrate that the Llano region was also episodically inundated by marine waters during the Devonian period. During periods of subaerial exposure, these deposits were largely stripped from the region of the Llano Uplift. The pockets and remnants of Devonian strata preserved in paleokarst included the Bear Spring Formation, Pillar Bluff Limestone, Stribling Formation, and, in part, the Houy Formation. Breccias found at the base of the base of the pockets of Devonian strata likely represent a mixture of residuum developed by the subaerial, in situ dissolution of underlying limestones and dolomites and residuum eroded and redeposited by an advancing marine shoreline. Late Paleozoic (Carboniferous and Permian) Like the Devonian strata found in the Llano Uplift, early Carboniferous (Mississippian) strata, the youngest black shale of the Houy Formation, the crinoidal limestone of Chappel Limestone and the black shale Barnett Formation consist of at most a few meters of strata preserved within collapse structures and other paleokarst. As in case of the Devonian and Silurian strata found within the Llano uplift, these strata represent brief periods of inundation of the region by shallow epicontinental seas and marine sedimentation alternating with long periods of terrestrial exposure during which these marine sediments were almost completely removed by erosion. Late Carboniferous ( Lower Pennsylvanian ) strata are in large part exposed in three non-contiguous areas. First, an isolated areal exposure of Smithwick Shale and underling Marble Falls Limestone occurs near Marble Falls, Texas, area in southwestern Burnet County. Second, in southwestern Mason County and northeastern Kimble County, late Carboniferous Marble Falls Limestone overlying relatively thin early Carboniferous strata is exposed within a half dozen, isolated fault blocks on the southwestern periphery of the Llano region. Finally, late Carboniferous are exposed as within a triangular shaped region that is bisected by the Colorado River along the northwest, north, and northeast periphery of the Llano