Theodore Roosevelt National Park - The Delicate Badlands

Despite the seemingly endless vistas of grass, despite the implacably rugged badlands that slash through them, despite the calm quality of eternal sameness that the Little Missouri lends as it winds northward toward its junction with the Missouri, despite all the elements that create the illusion of immutable toughness, this is a delicate, fragile, and ever-changing world. It is a landscape of limited water and shallow soil, altered by every rainstorm, remade with every spring thaw.

The gray clay that is largely responsible for the variable topography is called bentonite. It is the product of volcanic ash, which began settling over this land about 60 million years ago as the result of vast eruptions in the Yellowstone region, and continued to do so for tens of millions of years. Thick layers of it, intermixed with other sediments, lie beneath the topsoil, holding water for the roots of grasses and blocking the growth of most larger plants. Only the tough roots of the ubiquitous sagebrush, delving many feet deep for water, seem able to break easily through the barrier.

Each tiny particle of bentonite soaks up water and swells, doubling and redoubling its volume. When rain falls or snow melts, the top bentonite particles squeeze together into a slick shield through which water cannot seep. When this impermeable layer has absorbed as much water as it can hold, the rest runs off, cutting channels as it goes down through underlying layers of sandstone, mudstone, and clay. Soil that overlies the bentonite in these channels is eventually washed away, and the channels deepen into gullies and ravines.

Sometimes large masses of grassland slowly slide downhill, their paths lubricated by the slick bentonite beneath them. These are not landslides, broken and tumbling, but slump blocks: intact masses of soil held together by interwoven grass roots. They fill up low spots, leaving new slopes exposed; here the gray clay hardens, shrinks, and splits as it dries, forming deep cracks that will erode into new badlandscapes.

In many places the clay is protected from erosion by level patches of sod or by caprocks, including steel-hard pieces of petrified wood. The resultant pillars, cones, and castellated bluffs seem to grow ever taller as the gullies between them are eaten away, and their sides are striped and banded with the bright colors of underlying sediments -- brighter than in Badlands National Park because they came from the mineral-rich Rockies, rather than the Black Hills of South Dakota.

Contrasting with the bright badlands sediments are thick, black veins of lignite, or soft coal -- the partly rotted, partly petrified remains of palms, tree ferns, and other lush semitropical vegetation that flourished here some 60 to 70 million years ago. One sign of the coal's presence is black dust that sometimes peppers the pelts of prairie dogs when they emerge from their burrows. Another may be a horizontal band of Rocky Mountain junipers on a hillside; the trees' roots, blocked by clay beneath much of the badlands' dry, shallow soil, take moisture from the water-permeable lignite when they can reach it.

When the junipers take the shape of narrow columns, rather than spreading into graceful pyramids, it means that a nearby lignite vein has smoldered underground in the not too distant past. Such veins, ignited by lightning, grass fires, or even spontaneous combustion, may burn for decades. One vein, about 30 feet below the surface of the nearby Little Missouri National Grassland, is said to have been alight since before the first fur traders arrived. Heat from the burning lignite bakes the sediments above into a natural brick known as scoria.

At one time, the columnar junipers growing near these underground ovens were thought to be a separate species, but when such trees were transplanted away from the burning coal, they resumed their normal shapes. Exactly why the fumes cause junipers to grow this way no one knows, but the phenomenon has occurred elsewhere, downwind from coal-fired industries.

Today the land once again supports a rich and varied wildlife community. Bison graze the plains again; pronghorns -- North America's fleetest land animals -- feed largely on sage and wildflowers. Both white-tailed and mule deer are common, the whitetails leaping like ballet dancers through brushy thickets, the mules bouncing stiff-legged across open hillsides. Some 30 prairie dog towns -- one covering 106 acres -- support such predators as badgers, coyotes, and golden eagles.