Climbing the Rock Staircase: Three Parks, One Record of Time

Up the Staircase

1Last June, eighth grader Thandiwe Morrow and her aunt drove north through three parks in the dry Southwest: Sagebrush Canyon, Painted Steps and Hoodoo Rim. From a pullout on the highway, the land looked like a giant staircase. Each step was a band of colored cliffs, and the road climbed from one step to the next. Thandiwe kept notes at every stop, and later she combined them into Table 1.

2At Sagebrush Canyon, a river has cut a deep canyon down through flat layers of rock. Ranger Esperanza Quill explained that the layers formed one on top of another and have never been flipped over. At the very bottom, along the river, is a dark, twisted rock called schist, about 1,700 million years old. It has no layers like the rocks above it. Its top is an uneven, worn surface, and layer S2 rests directly on it.

3The ages in Table 1 come from volcanic ash beds and from fossils that geologists have dated at many places around the world. Some layers have no ash and no fossils that can be dated, so the table shows "?" for them.

4Painted Steps lies about 60 km north of the canyon. At its base is a cream limestone with the same sponge and brachiopod fossils as layer S5, the rim rock of Sagebrush Canyon. Only the top 30 m of this limestone shows at Painted Steps; its full 90 m shows in the canyon wall. Above it rise towering cliffs of tan sandstone with sloping lines called cross-beds, like the ones inside modern sand dunes. All 600 m of this sandstone shows at Painted Steps. Geologists estimate that it built up over about 10 million years.

5Hoodoo Rim, the last stop, is the highest point of the trip, about 2,500 m above sea level. At the foot of its cliffs, the top 40 m of the same tan dune sandstone shows. Above it lies gray shale with ammonite and oyster shells, which built up on a sea floor over about 20 million years. At the top is a pink limestone carved by rain and frost into tall spires called hoodoos. In the visitor center, Thandiwe saw snail shells and small mammal teeth that had been found in the pink limestone.