Middle Park Hydrogeologic Geologic Period Phase Stratigraphic Unit Unit Modern Alluvium and outwash deposits Alluvial Aquifer Quaternary Glacial deposits Glacial deposits Glaciation Older stream and outwash terrace Local perched deposits aquifer Grouse Mountain Basalt Volcanics Neogene Extension Troublesome Troublesome Formation Aquifer Rabit Ears Volcanics Volcanics Transition White River White River Formation Paleogene Aquifer Middle Park Middle Park Formation Aquifer Laramide Windy Gap Volcanic Volcanics Formation Middle ParkMiddle Member Pierre Shale Pierre confining Niobrara Formation unit Cretaceous Interior Seaway Benton Group Dakota Sandstone Dakota Aquifer Jurassic Morrison Formation Morrison Aquifer Mesozoic Triassic Sandstones Permian Ancestral Pennsylvanian Rocky Mountains Mississippian No strata Devonian Silurian Stable Ordovician Cambrian Crystalline rocks of igneous and Crystalline Precambrian Precambrian metamorphic origin in mountainous bedrock region Table 12a-06-02. Middle Park stratigraphic chart. Middle Park Unit Thickness Hydrogeologic Geologic Period Phase Stratigraphic Unit Physical Characteristics Hydrologic Characteristics (ft) Unit Well to poorly-sorted, uncemented sands, silts and gravels along modern Modern Alluvium and outwash deposits Alluvial Aquifer streams and as valley-fill Unstratified sand, gravel, and silt within, and at the mouths of, mountain Glacial deposits Glacial deposits Quaternary valleys Glaciation Older stream and outwash terrace Well to poorly-sorted, uncemented sands, silts and gravels on bedrock- Local perched deposits cored terraces above modern streams aquifer Grouse Mountain Basalt Basalt flow caprock Volcanics Neogene Extension Tuffaceous siltstone, fine-grained sandstone, glass shards, weathered ash, Troublesome Unconfined aquifer where tuffaceous siltstone is absent, confined where Troublesome Formation 0-1,000 and conglomerate Aquifer siltstone is present Interbeded tuff, tuff breccia and volcanic breccia interlayered with flows of Rabit Ears Volcanics 0 - 1,500 Volcanics intermediate composition Transition White River White River Formation Continental lakebed deposit Limited aquifer Paleogene Aquifer Middle Park Middle Park Formation 2,500-5,000 Sandstone, conglomerate and shale Aquifer Laramide Windy Gap Volcanic 0-700 Gray volcanic breccia, conglomearte and andesitic flows Volcanics Formation Middle Park Middle Park Member Pierre Shale 200-5,000 Dark-gray to brown shale, sandstone 1,500 feet above base Pierre confining Sandstone and limestone beds might yield limited water Niobrara Formation 400-500 Calcareous shale and limestone unit Cretaceous Interior Seaway Benton Group 0->300 Calcareous shale and limestone Lower sandstone, local basal conglomerate, middle shale, and upper Dakota Sandstone 140-400 Dakota Aquifer sandstone Jurassic Morrison Formation 25-400 Variegated clay shale and mudstone, sandstone at base and top Morrison Aquifer Limited aquifer Mesozoic Triassic Sandstones Permian Ancestral Pennsylvanian Rocky Mountains Mississippian No strata Devonian Silurian Stable Ordovician Cambrian Crystalline Precambrian Precambrian Crystalline rocks of igneous and metamorphic origin in mountainous region bedrock Table 12a-06-02. Middle Park stratigraphic chart, detailed. Colorado Geological Survey ON-010 Colorado Groundwater Atlas. Sources: Tweto (1957); Voegeli (1965); Izett (1966); Glover and others (1998); Kellogg and others (2008); Cole and Braddock (2009); Cole and others (2010); Raynolds and Hagadorn (2017).