Climbers rightfully take interest in the geology of the Tower. Worming their fingers into cracks and fissures, depending for their lives on the ringing-hard igneous mass that supports them, rock climbers develop a familiarity with various origins and kinds of rock. They, of course, contribute to the ongoing process of erosion which, over eons of time, has left Devils Tower in its present form. The amount of loose rock that breaks off under their feet, however, is far less than the amount lost every season from frost-heave, the effect of water seeping behind the surface rock and expanding into ice. The boulder field at the base of the Tower tells the rest of the erosion story. By studying lichen growing on the last slab to fall off the side, it has been estimated that this event occurred more than 10,000 years ago. Nearby National Parks Mount Rushmore National Memorial is 40 kilometers (25 miles) southwest of Rapid City, South Dakota. Patriotic ideals representing the birth of the Nation through its entry into 20th Century world affairs are symbolized in massive stone carvings of George Washington, Thomas Jefferson, Abraham Lincoln, and Theodore Roosevelt. The scale and difficulty of sculptor Gutzon Borglum’s achievement perhaps can only be appreciated in person. From June 1 to Labor Day the faces are illuminated at night and evening programs are presented in the amphitheater. Mailing address: Keystone, South Dakota 57751. Wind Cave National Park is located on U.S. 385, 16 kilometers (10 miles) north of Hot Springs, South Dakota. Wind whistling from the entrance attracted attention to the cave, hence its name. Jesse McDonald developed the cave as a tourist attraction in 1890. His son, Alvin, explored its passageways and named many of the fancy formations. Above ground the park preserves a natural prairie environment and a remnant bison herd, as poignant a scene as you will find anywhere in the West. Tours of the cave are conducted by park rangers throughout the year except on December 25 and January 1. A campground is open from mid-May through mid-September on a first-come, first-served basis. Mailing address: Hot Springs, South Dakota 57747. [Illustration: _Mount Rushmore is one of the most grandiose monuments of patriotism in the United States, attracting travelers from across the Nation and around the world._] [Illustration: _The bison herd at Wind Cave National Park conjures up images of days gone by when as many as 60 million of these animals roamed a 30-state area between the Rockies and the eastern woodlands._] Jewel Cave National Monument