Stretching across 9,000 square miles, the Absaroka Volcanic Field forms one of the largest volcanic fields in the continental United States. Between thirty-five to fifty-five million years ago, volcanoes in the Yellowstone region erupted and deposited 10,000 feet of lava and debris. Glacial activity later sculpted this volcanic material into the Absaroka Range seen today, including the castle-like forms nearby. Remote areas of the range can be accessed only by horseback or foot, making study, mapping, and development difficult. Fossils found in the deposits include tropical and subtropical plants and animals from the time of the eruptions, showing that the area has experienced dramatic climate change since the Absaroka volcanoes were active.