_Broken Log Sections._--Rhythmic vibrations of earthquakes during the period of uplift of the land are believed to have produced fine cracks at more or less regular intervals across the petrified logs. As erosion of the softer material around the logs took place at the surface of the land, the exposed cracks widened, separating the logs into sections.
_The Painted Desert._--The badlands of the Petrified Forest areas and the Painted Desert get their color from the ancient volcanic deposits of that region, and the surface forms are typical of desert erosion.
The material from which the badlands were sculptured originally was deposited layer upon layer as volcanic ash. The decomposition of the ash which released silica for petrification converted the ash into claylike rock, called bentonite. When pure, the bentonite is nearly white, but in the Painted Desert it is stained all shades of red, orange, maroon, blue, purple, and yellow by iron minerals that also came from volcanic ash.
[Illustration: _Interesting Formations in the Third Forest._]
Bentonitic beds in arid or semiarid regions erode into badlands. The bentonite absorbs water like a sponge, swells, and disintegrates into a fine mud. As a result, the torrential summer rains that fall in northern Arizona rapidly cut the banded, bentonitic beds into sharp, conical hills, turreted ridges, and sharp, interbranching canyons and ravines. When dry, the bentonite is hard and strong, thus preserving these intricate badlands forms during the long periods between rains. Locally, a hard sandstone caprock may prevent rapid erosion of the shales beneath to form an abrupt-sided, table-topped butte or mesa. The resistant capping of the rim of the Painted Desert is composed of ancient volcanic rock.
All visitors are invited to see the Rainbow Forest Museum. Its exhibits include many outstanding examples of polished petrified wood, fossils, and minerals; charts explaining the formation of the petrified forests and the badlands; and a diorama. Other exhibits may be seen at the Painted Desert Museum, which is open during the summer.
During the summer months short talks are given periodically in the Rainbow Forest Museum; and, as circumstances permit, guided tours through the Rainbow Forest are conducted by park ranger naturalists.