Except in the southern third of the monument where cindery pumice covers the surface, the visitor walks on solid lava. The small recent flows of billowy lava, or the pahoehoe type, have spread out like thick molasses or tar. It is in this type of lava that the caves and tunnels of the monument are found. These were formed by the hardening of the surface and sides of lava flows, becoming tubes when the molten lava core drained out. They range from a few feet to several miles in length and from 10 to 75 feet in height. Collapsed portions form long serpentlike trenches of broken rock 20 to 100 feet deep and from 50 to 250 feet wide, while narrow unbroken roof strips serve as natural bridges. The caves being of volcanic origin, lava stalactites are in evidence in some of them. Ice formations, the result of constantly freezing temperatures and presence of moisture, are found in others. Over 300 caves have been located in the monument, but of this number only 130 have been explored. A small number of the more interesting caves are open to the public. In exploring these caverns, visitors are given the services of guides without cost during the summer months. Two types of symbolic Indian writings add a touch of mystery to the monument, suggesting the presence of ancient aborigines many centuries ago. One type is made up of paintings (pictographs) on the walls of a number of the caves and sides of natural bridges. The other type is composed of carvings (petroglyphs) confined entirely to rocky bluffs where the material was sufficiently soft to be cut out by stone tools. The petroglyphs, located in an isolated portion of the monument, are deeply carved and have successfully withstood the ravages of time. Historical features of the monument recall the only major Indian war ever fought on California soil and one of the most costly of its kind in United States history. In 1872-73 a small band of Modoc Indians under the leadership of Keintpoos, commonly known as Captain Jack, clashed with a body of United States cavalry just north of the California line following a rampage during which several settlers were killed. A short time later, the Modocs established themselves in a natural lava fortress in the northern part of the monument where they withstood a superior force of soldiers for 5 months. During this time several peace negotiations were attempted, reaching their climax in an ill-fated peace parley a short distance from the Indian stronghold. A number of Indians, including Captain Jack, gathered with a peace commission, headed by General R. E. S. Canby, commander of the besieging soldiers. During the truce parley, the Indians without warning killed General Canby and one other member of the commission, after which the Modocs fled to their stronghold. A cross, erected by Canby’s soldiers on the spot, still stands.