The rugged hills to the north are among the oldest visible physical features on earth. Their hard quartzite rock was deposited as sand in a shallow sea that once covered this region. Although the layers were deposited horizontally, they were later warped until they were tilted fifteen degrees at Devils Lake and vertically at Rock Springs. The gorges at these places were eroded by rivers millions of years ago, and the Wisconsin River once flowed through the Devils Lake Gorge. Glaciers later brought debris that blocked the river, and after the ice melted a lake remained. Exploration for iron ore began in 1887, ore was found in 1900, and mining operations started. By 1925 mining in the district ceased because costs had become too high to compete with other sources, after producing 643,030 tons of ore.