Geologic formation field guide
Explore 36 comprehensive formation guides, balanced across six major U.S. regions. Every formation has an isolated, permanent page covering distribution, age, origin, rocks, fossils, minerals, resources, field context, and authoritative sources.
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Explore spatially
The overlay shows mapped geologic units from Macrostrat across the country. Select any colored area—or inspect the map center with a keyboard-accessible control—for source-map context. When LithoMap has completed an original formation profile, the selected unit links to its dedicated page.
The directory below remains available while the map loads.
The overlay is a blended visualization assembled from maps of different scales. Boundaries and names are approximate and should not be used to establish ownership, access, hazards, or collecting permission.
The foundation collection contains exactly six important units from each major region. Future additions will arrive in balanced 12-page batches—two per region—rather than as thin, automatically generated pages.
Region 1 of 6
Late Eocene to early Miocene
The John Day Formation is a long, ash-rich record of changing Oregon landscapes, preserving plants and mammals through a transition from warm forests to cooler, drier open environments.
Eocene
The Clarno Formation records Eocene volcanoes and subtropical forests in Oregon, where lava, ash, and lahars buried plants and animals and created unusually rich fossil sites.
Late Eocene to Oligocene
The Ohanapecosh Formation is a thick, altered volcanic succession beneath parts of the Washington Cascades, built by ancient eruptions, underwater debris flows, ash, and lava long before modern Mount Rainier formed.
Miocene
The Monterey Formation is a varied marine succession of siliceous, organic-rich, and carbonate rocks that records exceptionally productive Miocene seas along coastal California.
Early to middle Eocene
The Crescent Formation is an immense Eocene basaltic foundation of the Olympic Peninsula, assembled largely by submarine eruptions before the volcanic crust was attached to western North America.
Paleocene to early Eocene
The Siletz River Volcanics are a thick stack of oceanic basalt, pillow lava, and fragmental volcanic rock that forms much of the basement beneath the Oregon Coast Range.
Region 2 of 6
Late Jurassic
A broad sheet of continental sediment famous for Late Jurassic dinosaurs, the Morrison records shifting rivers and floodplains across much of the western interior before the modern Rocky Mountains existed.
Eocene
The Green River Formation is the layered archive of large Eocene lake systems in Wyoming, Utah, and Colorado, preserving exceptional fossils, organic-rich mudrocks, and an unusual suite of minerals formed in changing lake water.
Late Triassic
The Chinle is a colorful Late Triassic river-and-floodplain record of the American Southwest, celebrated for petrified wood and for fossils that document ecosystems long before the rise of familiar Jurassic dinosaurs.
Early Permian
The Kaibab forms much of the resistant rim of the Grand Canyon and records a shallow Permian sea whose limestones, dolostones, sandstones, chert, and gypsum capture repeated marine advances and retreats.
Late Eocene
The Florissant Formation records a high-elevation Eocene lake periodically disturbed by volcanic eruptions, with fine lake beds preserving insects and leaves while volcanic mudflows buried enormous redwood relatives.
Early Jurassic
The Navajo Sandstone is the rock record of an immense Jurassic sand sea, recognizable by sweeping cross-beds, pale cliffs, rounded domes, and a quartz-rich fabric that stores and transmits groundwater.
Region 3 of 6
Latest Cretaceous
Hell Creek records the last slice of dinosaur-dominated time in the northern Great Plains, when rivers crossed a low coastal plain east of the rising western mountains and near the retreating Western Interior Seaway.
Late Cretaceous
The Niobrara is a chalky marine unit deposited in the Western Interior Seaway, preserving abundant evidence of Cretaceous ocean life and forming pale cliffs, benches, and subsurface petroleum reservoirs across the central United States.
Early to early Late Cretaceous
Dakota is a long-used and regionally complicated Cretaceous rock name for sandstone-rich strata deposited as rivers and coastal environments shifted ahead of the Western Interior Seaway.
Late Cretaceous
The Pierre Shale is a vast Late Cretaceous marine succession of the Great Plains, known for gray shale, bentonite, fossil-bearing concretions, and pronounced engineering problems on clay-rich slopes.
Miocene to Pliocene
The Ogallala Formation is a widespread apron of river-borne sediment shed from the Rocky Mountains, forming much of the High Plains and containing the principal geologic material of the High Plains aquifer.
Latest Cretaceous
The Fox Hills Sandstone records the final retreat of the Western Interior Seaway across the northern Great Plains in shoreface, barrier-island, and tidal deposits rich in marine traces and shells.
Region 4 of 6
Middle to Late Ordovician
Trenton Limestone is a historically important Ordovician carbonate name applied to fossil-rich marine rock across parts of the eastern and interior United States, with modern rank and subdivisions that vary by basin.
Late Devonian to earliest Mississippian
The New Albany Shale is an organic-rich marine unit of the Illinois Basin whose dark beds record restricted seafloor conditions and whose lighter layers capture changes in oxygen and sediment supply.
Late Ordovician
The Maquoketa Shale is a widespread Upper Midwest marine succession of shale and carbonate beds, known for diverse fossils and for major changes in facies and stratigraphic rank across state lines.
Middle Ordovician
The St. Peter Sandstone is a notably quartz-rich Ordovician unit spread across the central United States, important as an aquifer, industrial-sand resource, and record of repeated marine reworking.
Late Cambrian
The Eau Claire Formation is a fine-grained Cambrian marine interval above the Mount Simon Sandstone, notable for trilobite-bearing beds and for acting as a regional groundwater confining unit.
Late Cambrian
The Mount Simon Sandstone is a thick basal Cambrian sandstone laid across ancient crystalline basement, serving today as a major deep aquifer and a target for subsurface fluid storage research.
Region 5 of 6
Middle Devonian
The Marcellus is an organic-rich Devonian marine shale of the Appalachian Basin, important both as a record of an ancient sea and as a deeply buried natural-gas reservoir whose name and rank vary across state lines.
Late Devonian
The Catskill Formation is a thick Appalachian red-bed succession built by rivers carrying sediment westward from rising Acadian mountains during Late Devonian time.
Late Devonian
The Lock Haven Formation is a fossil-bearing marine transition of shale, siltstone, and sandstone that records a Late Devonian shoreline advancing westward across Pennsylvania.
Early Mississippian
The Pocono Formation is a resistant Appalachian sandstone succession that records rivers, deltas, and shallow coastal settings near the beginning of Mississippian time.
Latest Ordovician to Early Silurian
The Tuscarora Formation is a hard, quartz-rich ridge former of the central Appalachians, deposited around the Ordovician–Silurian transition as sand spread across a changing shallow sea.
Middle Devonian
The Onondaga Limestone is a fossil-rich Devonian carbonate of the Appalachian Basin, forming a durable marine marker beneath younger black shales including the Marcellus.
Region 6 of 6
Late Eocene
The Ocala Limestone is a porous, fossil-rich Eocene carbonate spread across the Florida Platform and adjoining Coastal Plain, central to the region’s karst landscapes and groundwater system.
Oligocene
The Suwannee Limestone is an Oligocene marine carbonate beneath much of Florida and south Georgia, recognizable by fossil-rich limestone and important to regional groundwater and karst.
Middle to late Eocene
The Castle Hayne Limestone is a highly fossiliferous Eocene carbonate of eastern North Carolina, formed from shell, bryozoan, and lime sediment on the Atlantic shelf.
Pliocene
The Yorktown Formation is a shell-rich Pliocene marine deposit of the Atlantic Coastal Plain, preserving a diverse record of warm shallow seas before modern Chesapeake and Carolina landscapes formed.
Early to middle Miocene
The Calvert Formation is a fossil-rich Miocene marine unit exposed spectacularly along Chesapeake Bay, recording changing shelf depths, productive waters, and repeated sedimentary cycles.
Pliocene to possible early Pleistocene
The Citronelle Formation is a weathered blanket of sand, gravel, silt, and clay across Gulf Coastal Plain uplands, recording large late Neogene river systems and a still-debated age.
