In 1874, US Army commander George A. Custer led a group into the Black Hills area. He and some of his officers climbed the crest now called Black Elk Peak, and made a toast to US General William S. Harney. They named the peak for Harney, and this name was used until 2016. This expedition reported that gold could be found in the Black Hills, which spurred a gold rush into the future county area. The mining settlements that sprang up were in violation of a treaty signed with the Sioux Nation in 1868. By 1875, settlement was sufficient to justify creation of a local governing organization, so Pennington County was created on January 11, 1875, and it was fully organized by April 19, 1877. The county's boundaries were adjusted in 1877 and in 1898.