About 350 million years ago, much of Montana was submerged under a shallow sea where billions of tiny marine creatures lived, and after they died their bodies settled into seabed muck that, after hundreds of millions of years of accumulation and many more millions of years, metamorphosed into the pale gray rock known today as Madison Limestone. This limestone is common throughout Montana, eastern Idaho, northern Wyoming, and the Dakotas, and in Montana its beds are from 1,000 to 2,000 feet thick in places. Because Madison Limestone resists weathering and erosion much better than most other kinds of rock, it forms many of the spectacular cliffs and dramatic ridges in Montana, including limestone pinnacles exposed when surrounding soil eroded away and created the dramatic canyon along the Clark Fork River; the red streaks visible on the rocks and soil are iron oxide. About 75 million years ago, molten rock intruded the area near the crest of the Garnet Range, and northwest-trending faults and rock layers channeled mineral-rich fluids from the intrusion into Cambrian and Precambrian rocks to form three principal gold veins and numerous smaller gold-bearing zones. Prospectors discovered gold placers at the mouth of Bear Gulch in 1865, and discoveries in other drainages of the Garnet Range soon followed. Although gold-bearing veins were discovered in 1866, the technology to work them was not readily available, but by 1896 numerous underground mines were producing gold, silver, and copper. In 1898, more than 1,000 people lived in the town of Garnet to support miners in the surrounding area. Where magma contacted the Madison Limestone, it caused a chemical reaction called a skarn that formed the garnets found in the range. The Garnet area placer mines produced about 60,000 ounces of gold, as did the lode mines, and drilling has revealed gold placer reserves under the current rest area while resources of several hundred thousand ounces of gold still remain in the Garnet Range. A mining camp called Beartown stood in a narrow gulch nearby, and between 1865 and 1869 miners recovered $30 million in gold and silver from Bear Gulch; as many as 7,000 people lived in the camp during its heyday.