PINNACLES National Monument Remnants of an ancient volcano are carved into rugged scenery by Nature's weathering agents—wind and rain, heat and cold. THE MONUMENT was established in 1908 by Presidential proclamation. It is unique for its mass of spires and crags that rise 500 to 1,200 feet against the sky. Here is dis- played the remains of a volcanic mountain formed by massive lava flows and violent ex- plusive eruptions millions of years ago. From this ancient volcano, erosion has carved a rugged pinnacle area of some 2 square miles. The canyon areas of Bear Gulch and Chalone Creek contain caves or “covered can- yons” formed by large blocks of rock which have fallen from the steep walls. David Starr Jordan visited the area fre- quently and was instrumental in its selection as a National Monument. Tiburicio Vasquez, a daring and notorious bandit, is said to have taken refuge among the caves and crags of Pinnacles in the latter part of the 19th century, before finally being brought to justice. The monument contains 23 square miles, and is 4 miles at its greatest east to west width and 7 miles long north to south. The pinnacles are volcanic rocks, and they mark a spot that roared with explosive erup- tions in Tertiary time, some 30 million years ago. Weather and erosion have since removed the numerous craters and about two-thirds of the volcanic mountain they had built. In the worn-down base, five old vents are found through which explosive eruptions threw the viscous lava and broken stone high into the air and loaded the steep slopes of the volcano with accumulations many thousands of feet in thickness. Stone blocks 10 feet in diameter were shot forth by the escaping steam and hot gases, but the violence broke most of them into frag- ments only a few inches thick. Liquid lav- was sprayed into the air and fell as fine dus- the so-called volcanic ash, in which the broken fragments are embedded. The mixture is known as volcanic breccia. Among the fragmental layers are also beds of solid lava, showing that quiet flows sometimes alter- nated with the explosive eruptions. These vast accumulations have been eroded from the summit area, and Chalone Creek has swept them all away from the eastern flank of the old volcano. Beds many thou- sands of feet in thickness that still remain from the old western slope have been carved into the scenic pinnacles, cliffs, caves, and canyons. The central core of the volcano be- The Geologic Story This rugged area is wholly different from the surrounding country which has the smooth ridges and valleys of the coast ranges. NATIONAL PARK SERVICE DEPARTMENT OF THE INTERIOR The National Park System, of which this area is a unit, is dedicated to conserving the scenic, scientific, and historic heritage of the United States for the benefit and enjoyment of its people.