In 1864, prospectors discovered promising placer gold deposits in Confederate Gulch, named for local political sympathies during the Civil War, and a major gold rush followed that brought thousands of people to the gulch and revealed some of the richest placer deposits in Montana. The gold came from gravel in Confederate Creek and from gravel bars above the creek laid down during the Pleistocene Epoch, and gold from Montana Bar, a 2-acre gravel deposit on a bench above the gulch, was so abundant that it clogged miners' sluice boxes, with some miners said to have taken $15,000 to $21,000 in modern currency per pan. The bonanza lasted only three years, and by 1900 the gulch was nearly deserted, but Confederate Gulch produced nearly $150 million in gold. The gold-bearing gravels overlie the Precambrian Spokane, Greyson, and Newland Formations of the Belt Supergroup, deposited in an inland sea more than a billion years earlier, and these ancient rocks were brought to the surface by large thrust faults that stacked older rocks over younger and formed the Big Belt Mountains. Confederate Gulch is crossed by a line of igneous plutons, including Miller Mountain, Boulder Baldy, and Mount Edith, formed from a single magmatic event when magma intruded the Belt Supergroup rocks between 69 and 74 million years ago and cooled before reaching the surface; the pluton on Miller Mountain was extensively prospected as a lode gold source. In December 1864, ex-Confederate soldiers built log cabins beside the gulch later named for them, and the snow paths between the cabins formed a diamond pattern that gave Diamond City its name. After news of the rich deposits spread beyond the Big Belts, at least 5,000 people came to Diamond City, which for a short time served as the seat of newly created Meagher County and had stores, saloons, hurdy-gurdy houses, and hundreds of log cabins, though nothing of the camp remains today.