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The Cooke City area is distinctively located at the intersection of several of North America's major geologic and tectonic features. This is where the uplifted Beartooth Plateau meets the northern end of the Absaroka Mountains volcanic chain and the northeast margin of the Yellowstone Super-Caldera. Yellowstone itself is the eastern most extension of a 17 million year migration of the Snake River Plain, a 700 kilometer long intercontinental rift filled with volcanics. A fourth unique geologic feature is the enigmatic Heart Mountain Detachment Fault that began in the Valley of Silver Gate and Cooke City.
The Beartooth Plateau, immediately north and east of Cooke City, is a fault-bounded, uplifted block of ancient granites that are almost 3 billion years old. This 30 by 60 mile block rises abruptly above the Great Plains which bound it to the east and north. A spectacular drive over Hwy 212 to Red Lodge shows an ancient flat terrain, now incised by younger erosion, which probably looks much like it did over 500 million years ago just before life on earth exploded in the seas that rose to cover this ancient plain. The plateau rises to over 11,000 feet on Highway 212 with many peaks over 12,000 feet in elevation.
The Yellowstone Super-Caldera is the result of volcanic explosion so rare and large that it occurs only once about every hundred thousand years globally. A hot plume of magma rising from the mantle caused this large violent eruption which was over 30 miles wide and lifted so much rock and dust that it covered distant places, such as Denver Colorado, with over 6 feet of dust. Closer to the park it deposited molten rock, hundreds of feet thick, upon the earth.
The frequency of major eruptions at Yellowstone is about every 600 to 700 thousand years. The last such event here was 640,000 years ago; however there is no sign that another such event will happen for many thousands of years, so enjoy a carefree visit here.
The Heart Mountain Detachment Fault is a major flat, gravity-driven fault that slid south starting about 3 miles west of Cooke City. It is unusual because it slid on a single flat plane almost 2000 feet under the surface. The overlying 2000 feet of rocks that were carried southward on this fault broke up into many pieces, like icebergs in the sea, and left deep canyons and crevices down to the denuded surface before fresh volcanics filled in the gaps.
This flat detachment surface can be easily seen on the south side of Cooke City and along the southern side of Hwy 212 for about 20 miles. The prominent white ledge is the ancient Cambrian Pilgrim Limestone; the detachment fault lies about 18 feet above this ledge and continues almost 50 miles south to Heart Mountain near Cody Wyoming. Remarkably, it may have taken only a few hours to a day's time to move 50 miles. Why it broke where it did and what the mechanism was to float such massive amount of rock upon a gently sloping plane (~10 degrees southern dip) remains controversial.
The Absaroka Range was formed about 45 to 55 million years ago by active and long term volcanism. Cooke City is at the very northern tip of this high mountain chain which defines the eastern edge of Yellowstone Park. Scotch Bonnet Mountain, at nearby Lulu Pass, is a diorite stock which may be the frozen magmatic root of a large stratovolcano that formed much of the volcanics surrounding the Cooke City area.
Called Galena and Shoofly in early years, Cooke City was founded as a mining town. Minerals were discovered in 1869 by four trappers, fleeing from Indians. Bart Henderson probably traveled here in 1870 on a scouting trip. The first mining claim was recorded by Adam Miller 1871, when the area officially became the New World Mining District. One year later Yellowstone was made a national park, and the park boundaries were drawn to exclude the known mining.
Magmatic intrusive rocks intruded the Cambrian sediments during the early formation of the Absaroka Mountains, 50 million years ago. Beneath thick volcanics, these limestone rich sediments reacted with the rising, acidic and metal-rich solutions from these intruding magmas. After uplifting and erosion, the resulting deposits and small mines were spatially zoned, with lead, silver and zinc deposits further away and gold and copper in a large breccia pipe beneath Henderson Mountain, the center of activity near Daisy Pass.
Mining, primarily for lead, silver and zinc, was sporadic until the early 1990's. Chief Joseph reportedly acquired lead for bullets from the Republic Mine, just south of Cooke City, during his historic escape attempt from the U.S. Cavalry in 1877. Gold was mined at the McLaren Mine near Daisy Pass from 1933 until 1953. In 1990 a major gold, silver and copper deposit was discovered lying beneath nearby Henderson Mountain on private land. This underground deposit contains over 2 million ounces of gold, 5 million ounces of silver and 700 million pounds of copper. Because this deposit is so close to Yellowstone Park, in 1996 President Clinton withdrew the surrounding 22,000 acres from public use for at least 20 years, making mining untenable.
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