ROCK BISCUITS

By Dr. Chas. N. Gould,
Former Regional Geologist.

Rock biscuits, done to a golden brown, some of them 20 feet high and 30 feet in diameter, are "served" to visitors as one of the unique attractions in Petit Jean State Park, Arkansas, about 50 miles northwest of Little Rock. Nothing quite like them has been found elsewhere. Thousands of people see them each year, and wonder how they were formed. What was Mother Nature's recipe for making this pan of biscuits, and how did she manage to get that brown tint on them? Rather complicated geological processes were involved in forming and "baking" these dome-shaped masses of sandstone.

Most of the hard rocks exposed on the earth's surface contain cracks or crevices. Geologists call them joints. Usually joints are arranged in a definite pattern. In the case of these rock biscuits there are two systems of joints crossing nearly at right angles, forming squares. The joints were probably formed by shrinking of the earth's crust. Then water sinking along the joints dissolved portions of the rock, allowing other portions to crumble away, and finally oval-shaped surfaces like the tops of biscuits were formed.

Mother Nature did not use a giant biscuit-cutter to shape the rock biscuits. Neither did she bake them in a fiery furnace. Instead, the cool water that trickled over the sand grains contained iron in solution; part of the iron was deposited as cementing material in the sandstone, and when exposed to the atmosphere, the iron oxidized to give the varying brown tints. With slightly different physical conditions the sandstone probably would have weathered out into stone pillars, pinnacles, towers, balanced rocks, and other erosion forms, such as in Bryce Canyon National Park, Utah; and Chiricahua National Monument, Arizona.