The quantity, quality, and distribution of surface water and groundwater in Wyoming are intimately related to the regional patterns of rock units (Plate 4A), associated geologic structures, and major topographic elements (Plate 4B).

The Precambrian rocks (ranging from 3.9 to 1.4 billion years in age) are crystalline and metamorphic types such as gneisses, schists, quartzites, and granites. The Paleozoic and Mesozoic rocks (from 570 to 65 million years old) are dominantly sedimentary strata such as limestones and dolomites, sandstones and shales, including red beds. The Tertiary rocks (from 65 to 2 million years old) include various sedimentary types, such as conglomerates and sandstones, siltstones and claystones, as well as volcanic and intrusive igneous rocks. The Quaternary materials (of the last 2 million years) are largely unconsolidated sedimentary deposits containing particles which range in size from boulders to clay. In the northwest part of the state, igneous rocks of Quaternary age are composed largely of pyroclastics and some lavas.

The typical Wyoming mountain structures (except those in the western part of the state) are delineated by elongated map patterns for Precambrian rocks, typically flanked on one side by bands representing the Paleozoic and early Mesozoic rock sequences which may be thinner or missing along the other side. These mountains include: the Wind River and Gros Ventre, Bighorn-Owl Creek, Laramie, Medicine Bow, and the Sierra Madre. Structurally, they are broad backed, Precambrian-cored anticlines flanked by outward dipping Paleozoic and Mesozoic strata on one side and bounded by low angle reverse faults (i.e. thrust faults) on the other side. This structural style characterizes the Laramide Orogeny of latest Cretaceous-Eocene age.

The Wind River, Gros Ventre, Bighorn, Sierra Madre, and Medicine Bow ranges are classified geomorphically as high mountains with less than 20 percent gently sloping terrain and local relief between 3,000 and 6,000 feet. Portions of these ranges were heavily glaciated during the Pleistocene epoch (1.6 million to 10,000 years ago), and the Wind River Range continues to support glaciers (see Chapter 7). The Owl Creek Mountains are characterized as low mountains with less than 20 percent gently sloping terrain and local relief between 1,000 and 3,000 feet. The Laramie Range is classified as open low mountains with 20 to 50 percent gently sloping terrain, vertical slopes between 1,000 and 3,000 feet and 50 to 75 percent of the gentle slope occurring in lowlands.

Two other types of mountain structures are recorded along the western margin of the map. The Overthrust Belt, along the state boundary with Utah and Idaho, is shown by a pattern of closely spaced bands representing only Paleozoic, Mesozoic, and some Tertiary strata. Here in marked contrast to the broad Precambrian-cored Laramide uplifts, this orogenic belt contains a series of elongate ridges that are separated by relatively narrow valleys. The structure is characterized by folded strata within great imbricate slabs that were thrust eastward along a series of low angle faults. This style of deformation, contemporaneous with the Laramide, is referred to as the Sevier type. Topographically, the Overthrust Belt is classified as open low mountains with 20 to 50 percent gently sloping terrain, vertical slopes between 1,000 and 3,000 feet, and more than 75 percent of the gently sloping terrain occurring in lowlands.

The Tetons, a third mountain type, are represented on the state map as Precambrian rocks flanked by Paleozoic rocks on the west side, on the east the Precambrian is directly adjacent to late Tertiary rocks. The range is a great west-tilted block bounded on the east by a major normal fault of which the downthrown side forms Jackson Hole. Compared to other Wyoming mountains, the topographically impressive Tetons are of limited areal extent - some 35 miles long - between the Overthrust Belt to the south and the Yellowstone volcanics to the north. The Tetons, Wyoming's youngest mountains, are a post-Laramide structure originating in Miocene time. They may still be rising as Jackson Hole subsides. These mountains support several small glaciers and are geomorphically classified as high mountains with less than 20 percent gently sloping terrain and local relief between 3,000 and 5,000 feet.

The Yellowstone and Absaroka plateaus in northwestern Wyoming are major areas of volcanic rocks and volcaniclastic deposits that occupy and are adjacent to large calderas. However, the rocks in the two areas differ markedly in age. Those in the Yellowstone area, which is still underlain in places by chambers of molten material, are mainly Quaternary in age. In the topographically higher and more deeply dissected Absaroka area, the rocks are mainly Eocene in age, having erupted some 40 million years ago. Topographically the Yellowstone and Absaroka plateaus are classified as tablelands with high relief, 50 to 80 percent gently sloping terrain, local relief between 1,000 and 3,000 feet, and 50 to 75 percent of the gently sloping terrain occurring in uplands.

Although Wyoming is a mountainous state, some 70 percent is occupied by intermontane and piedmont plains. Their extent is indicated by the distribution of the Tertiary and Cretaceous rocks on the state geologic map. The plains are developed across thick sequences of sedimentary rocks preserved in the broad and deep synclinal basins that developed between the Laramide anticlinal mountains. The Cretaceous rocks, largely relics of pre- Laramide deposition, are overlain by Tertiary rocks that accumulated in the structural basins during and after the Laramide crustal deformation of latest Cretaceous through Eocene time.

In the Great Plains province the Wyoming segment is underlain by two broad structural downwarps: the Denver-Julesburg basin beneath the High Plains east of the Laramie Mountains and the structural and topographic Powder River Basin between the Bighorn Mountains and the Black Hills (a Laramide domal uplift within the Great Plains). The High Plains are developed across Miocene rocks, dominantly sandstones. Topographically, they are described as irregular plains or plains with low hills with 50 to 80 percent gently sloping terrain and local relief between 100 and 500 feet. The Powder River Basin is mainly floored by late Cretaceous shales; Paleocene sandstones; and Eocene siltstones, claystones, and some sandstones. The land surface form of the Powder River Basin ranges from plains with hills (50 to 80 percent gently sloping terrain, local relief between 300 and 500 feet, and 50 to 75 percent of the gently sloping terrain occurring in lowlands) to open high hills (20 to 50 percent gently sloping terrain, local relief between 500 and 1,000 feet, and 50 to 75 percent of the gently sloping terrain occurring in lowlands).

Within the mountain region, the broad Bighorn Basin, encircled by the Bighorn-Owl Creek Mountains and the Absaroka Plateau, is largely floored by Eocene deposits, claystones and siltstones with some sandstones. The Wyoming Basin, extending southward from the Bighorn-Owl Creek Mountains, is a complex region containing several synclinal basins separated by low Laramide anticlinal structures. The Laramie and Shirley basins are largely floored by late Cretaceous and some Eocene rocks. The Hanna Basin, relatively small in area but structurally deep, is mainly floored by Paleocene rocks. The Wind, Green, Great Divide, and Washakie structural basins are largely floored by Eocene strata. The Saratoga Valley is mainly floored by Miocene sandstones.

Anticlines within the Wyoming Basin include the Casper arch and the Rock Springs uplift, both of which are breached to expose cores of late Cretaceous shale rimmed by outward dipping Cretaceous sandstone ridges. Precambrian rocks are exposed in the core of the Rawlins uplift which, although small, is comparable in structure to the major Laramide anticlinal mountains. The Granite Mountains represent a major Laramide structure, deeply eroded to expose a Precambrian core that subsequently subsided during Miocene and later time along normal faults that produced the graben now traversed by the Sweetwater River. The graben largely contains Miocene sandstones surrounding exfoliation domes of Precambrian granite.

The regional geologic history of Wyoming strongly influences the direction of the stream courses and the nature of the surface and underground waters. A special feature reflecting the late Cenozoic history is the prevalence of rivers that flow from basin floors into canyons, cut across the adjacent mountains, and flow onto adjacent plains. This phenomenon results from the superposition of the streams (Plate 4C). The stream courses were initially established in late Miocene time when the basin had been largely filled by Tertiary deposits that locally extended across lower segments of the mountain axes. During the subsequent regional excavation of the basin fills, which began about five million years ago (in latest Miocene time), the streams maintained their courses across the mountains, cutting the transverse canyons. Topographically, the Bighorn and Wyoming basins are classified as plains with high hills (50 to 80 percent gently sloping terrain, local relief between 500 and 1,000 feet, and 50 to 75 percent of the gently sloping terrain occurring in lowlands). Badlands topography has developed in portions of the Bighorn Basin where the Willwood Formation is exposed. Extremely high sediment yields from this region lead to problems with turbidity in otherwise productive streams.

The quality of stream and underground waters in Wyoming is closely related to the regional distribution and nature of rock types. In the mountains where the rocks are dominantly Precambrian crystalline igneous and metamorphic types (along with some Paleozoic sedimentary strata), the stream and alluvial waters are characteristically of good quality and low in soluble materials. In the basins, the water in streams and especially from wells varies markedly in quality and abundance. Groundwater obtained from aquifers in Miocene strata of the High Plains has been successfully exploited for household use and irrigation. However, waters extracted from earlier Tertiary deposits and especially Cretaceous shales are, if obtainable, commonly high in a variety of soluble components and, therefore, unacceptable for domestic, irrigation, or stock use. Thus along with the climate of Wyoming, which is locally influenced by the mountainous terrain, geology is a major natural factor controlling the quality and availability of water throughout the state.

Plate 4A was digitized from Renfro and Feray (1972).
Plate 4B was digitized from Blackstone (1988).
Plate 4C was originally drawn by S.H. Knight.

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