During the expedition, William Clark often made side trips to climb hills for better views of the surrounding country. On April 22, 1806, he climbed Haystack Butte, the highest hill on that side of the river, and from its summit he had an extensive view and could plainly see the mountain range running south from Mount Hood, the Cascade Range. From that higher viewpoint, Clark could clearly see Mount Jefferson. The explorers had first seen the snow-covered peak on March 30, 1806, and named it for President Thomas Jefferson. Mount Jefferson stands 10,195 feet high and is one of 13 major volcanic peaks in the Cascade Range. Other peaks seen by Lewis and Clark included Mount Rainier, Mount St. Helens, Mount Adams, and Mount Hood. Although Mount Jefferson has not erupted in approximately 15,000 years, geologic evidence indicates that large, violent eruptions occurred there in the past. The Cascade Mountains extend from northern California to southern British Columbia and include many snow-capped volcanoes, some still active; Mount St. Helens erupted in 1980, and the U.S. Geological Survey continuously monitors seismic activity in the range.