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Greybull Sandstone Photo by David Lopez, MBMG, 2003 January July S MTWThF S S MTWThF S 1 23 123 4 5678 910 4 5678 910 11 12 13 14 15 16 17 11 12 13 14 15 16 17 18 19 20 21 22 23 24 18 19 20 21 22 23 24 25 26 27 28 29 30 31 25 26 27 28 29 30 31 February August S MTWThF S S MTWThF S 1234567 1234567 8 91011121314 8 91011121314 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 29 29 30 31 March September S MTWThF S S MTWThF S 123456 1234 7 8 910111213 5 6 7 8 91011 14 15 16 17 18 19 20 12 13 14 15 16 17 18 21 22 23 24 25 26 27 19 20 21 22 23 24 25 28 29 30 31 26 27 28 29 30 April October S MTWThF S S MTWThF S 123 12 4 5678 910 3 4 5678 9 11 12 13 14 15 16 17 10 11 12 13 14 15 16 18 19 20 21 22 23 24 17 18 19 20 21 22 23 25 26 27 28 29 30 24 25 26 27 28 29 30 31 May November S MTWThF S S MTWThF S 1 123456 234 5678 7 8 91011 12 13 91011 12 13 14 15 14 15 16 17 18 19 20 16 17 18 19 20 21 22 21 22 23 24 25 26 27 23 24 25 26 27 28 29 28 29 30 30 31 June December S MTWThF S S MTWThF S 12345 1234 678 9101112 5678 91011 13 14 15 16 17 18 19 12 13 14 15 16 17 18 20 21 22 23 24 25 26 19 20 21 22 23 24 25 27 28 29 30 26 27 28 29 30 31 Butte Office Billings Office 1300 W. Park Street, Butte, MT 59701-8997 1300 North 27th Street, Billings, MT 59101 406/496-4180 Fax: 406/496-4451 Montana Bureau of Mines and Geology 406/657-2629 Fax: 406/657-2633 Introduction Greybull Sandstone This years Montana Geology calendar features hoodoos in the Greybull Sandstone. The name Greybull originates from Grey Bull, a chief Hoodoos are pinnacles or pillars of rock formed by differential weathering and erosion of the Crow Tribe (fig. 3). In the early days of settlement in layered formations that have vertical variations in erosional resistance. Relatively in the northern Bighorn Basin, Grey Bulls band of Crows resistant rock forms the cap that protects the less resistant underlying layers. These spent the winters in the area of present-day Greybull, hoodoos are located about 8 miles almost due west of Pryor, Montana. Here, the red Wyoming. Because of friendships that developed between color in the upper layers of the hoodoos indicates that relatively high concentrations of the Crows and the early settlers, when the population grew iron oxide bind the sand grains together. In contrast, the tan beds below are less well large enough for the organization of a town it was named cemented. The erosional process typically begins by chemical weathering along intersecting in honor of Chief Grey Bull. vertical joints or fractures. The weathered and poorly cemented parts of the rock are more easily eroded, and thus the fractures deepen and widen until the rock remnants left between Figure 3. Chief Grey Bull. Reprinted with permission the fractures are the hoodoos seen here (fig. 1). from Hardin Photo Services, Hardin, Montana. The stratigraphic name Greybull Sandstone was first a drillers term applied to Figure 1. Erosional development of hoodoos from rock layers having subsurface oil-bearing sandstones discovered near the town of Greybull, Wyoming, in more cemented and resistant layer differing resistance to erosion. 1907. The Greybull Sandstone occurs at the top of the Kootenai Formation (fig. 4) and less cemented and resistant layer Origin of the Name Hoodoo most workers in the past have included it 1 The word hoodoo appears to have originated as part of the Kootenai Formation (or in African cultures and was used to refer to various Cloverly Formation in Wyoming). forms of magic and spiritual and medicinal healing. The Greybull Sandstone is an excellent The connotation of magic carries over into the oil and gas reservoir that has produced Thermopolis Shale Fall geological use of the term hoodoo, as their often bizarre tremendous volumes of oil and gas in several River SS 2 shapes commonly resemble the shapes of animals that fields in the Bighorn Basin (including the could embody evil spirits (Bates and Jackson, 1980). Greybull Field, which was the original Greybull Sandstone The word voodoo, used to describe spiritual practices discovery), and continues to be an important 14498 m.y.a. well known in the southeastern United States and oil and gas exploration target. In many areas LOWER CRETACEOUS Caribbean Islands, is an obvious derivative of the the Greybull is an excellent aquifer, providing 3 African hoodoo. potable ground water to one of Montanas most arid regions. In an area 2 to 10 miles Kootenai Formation Early History of the Region north of the hoodoos, several new wells The Crow Tribe (Absarokee) arrived in the area in the early 1700s after splitting drilled since 2000 are producing fresh water Pryor Conglomerate from the Hidatsa, then residing in the area of present-day Mandan, North Dakota. Before from Greybull Sandstone. this, the area was occupied by various tribes including the Shoshoni, Arapaho, Kiowa, The rocks forming the hoodoos shown 100 feet and perhaps Comanche. Because this part of south-central Montana has always been an on the calendar front were deposited as excellent hunting ground, several tribes periodically fought with the Crows for occupation sands in an ancient river channel that has of this rich land. Most commonly the Crows battled the Blackfeet, Sioux, Arapaho, and Shale been traced on the surface for at least 60 Sandstone Cheyenne. The trails used by the Native Americans commonly became the trails and miles, from Soap Creek Oil Field to just 163144 m.y.a. UPPER JURASSIC Mudstone roads of later travelers and settlers. Morrison Formation northwest of the town of Pryor, Montana Conglomerate Some historians suggest that the Verendrye brothers, Francois and Louis, reached (fig. 5). Channel sandstone in this system the foot of the Pryor or Bighorn Mountains in January 1743 while seeking the western is generally about 100 feet thick. Figure 4. Stratigraphic relationships of the Kootenai sea. The absence of undisputed landmarks in the journal of their trip, however, leads Exposures of Greybull Sandstone Formation and Greybull Sandstone. to multiple interpretations regarding the mountains they described. typically exhibit excellent planar cross From July 19 to 23, 1806, William Clark camped on the Yellowstone River less bedding (fig. 6a). Cross bedding is a natural phenomenon caused by the downstream than 20 miles northwest of the hoodoos while his men made two small canoes for their transport and deposition of sand. Figure 6b illustrates how cross bedding forms. The exploration downriver. On July 25 Clark passed the mouth of what the Crow had called direction of stream flow (paleocurrent) in the ancient Greybull channel can be determined Arrow Creek and named it after one of the Expeditions sergeants, Nathaniel Pryor. by measuring the orientation of the cross beds. Pryor and three men had been assigned to take the Expeditions remaining horses to Fort Greybull channel N These measurements indicate that about 100 Mandan (North Dakota) while Clarks party floated the Yellowstone. Pryors detail Fromberg Greybull exposures million years ago, the ancient river that formed managed to lose the horses and, at Pompeys Pillar, had to make bullboats to descend Fault Zone faults 5 miles the channel from Soap Creek to Pryor flowed the Yellowstone. These fragile craft, built of willow sticks and covered with buffalo hides westward and then changed abruptly tightly sewn together, generally were used by the natives to cross a river, not to float northward, as also suggested by the trace of Soap Cr. downriver some 350 river miles. Clark must have thought Pryors misfortunes rated a Pryor Oil Field the river channel (fig. 5). Surface geologic Little river being named for him. Mtn mapping has identified a system of faults in MOUNTAINSPRYOR Clark considered the Bighorn and Pryor Mountains to be a single range, and the MOUNTAINSBIGHORN the area of the channel bend. The abrupt name Pryor Mountains didnt come into general use until well after the 1870s. The Nye-Bowler northward change in the trend of the channel Crow are said to have used the name Arrow or Arrowhead Mountains because of the Fault Zone coincides with the location of these faults, chert they contain. Chert is a very fine-grained form of quartz that a skilled flint MT which suggests the faults were active and knapper easily can turn into projectile points, knife or hatchet blades, hide scrapers, WY controlled the direction and location of the or other cutting instruments. Figure 5. Structural control on Greybull channel rivers channel (fig. 5). The discovery of gold at Bannack, Montana, in 1862 and Virginia City in 1863 trend and Greybull channel exposures in the By studying surface outcrops and brought thousands of miners and people to southwest Montana. The principal route to Soap Creek-Pryor area. Arrows indicate understanding the factors that controlled the transport directions, measured from planar the gold camps was the Oregon Trail from Fort Laramie (along the North Platte River, cross bedding attitudes.