A bluff is a type of broad, rounded cliff. Most bluffs border a river, beach, or other coastal area.
Bluffs may form along a river where it meanders, or curves from side to side. River currents on the outside of the curve erode, or wear away, the lower part of a river bank. No longer supported, the upper part of the bank breaks off, leaving the high wall of a bluff. The 150-meter (500-foot) Great River Bluffs in the U.S. state of Minnesota, for example, were carved by the meanderings of the mighty Mississippi River.
Erosion also produces bluffs along the wider floodplain of a river. Over thousands of years, a meandering river gradually shifts from side to side across its floodplain. Where the meanders, or loops, of the river reach valley walls, the water may carve bluffs. In fact, a “bluff line” defines the outer limits of a river’s floodplain, and is often another name for valley wall. A floodplain’s bluff lines may be steep and narrow, or they may be wide and gentle.
Coastal bluffs are formed through a combination of erosion from wind, sea spray, and crashing waves. These bluffs are often more rugged than their inland counterparts, and are more vulnerable to major erosion. Coastal bluffs, especially those in the Puget Sound region of the U.S. state of Washington, are sometimes called feeder bluffs. The constant erosion of feeder bluffs supplies (feeds) sediment to the beaches and seashore downstream below.
Another sort of coastal bluff is the beach ridge. Beach ridges are formed entirely by waves lapping onshore, pushing sand and sediment up and away from the body of water. Beach ridges run parallel to the shoreline and are often associated with sand dunes. The Indiana Dunes, for example, are extensions of beach ridges formed by the waters of Lake Michigan. At this national lakeshore, tiny bluffs give way to larger dunes, and the ecosystem eventually creates the ideal conditions for an oak forest through the process of plant succession.
Like many types of cliffs, bluffs provide important information about how Earth developed. Scotts Bluff, for instance, rises more than 1,400 meters (4,600 feet) above the North Platte River in the U.S. state of Nebraska. The exposed rock of this national monument allows geologists to peer more than 30 million years into North America’s past. Rock formations at Scotts Bluff preserve the history of ancient volcanic activity nearby, as well as the fossilized presence of prehistoric North American rhinos, tapirs, and even camels.
Life on the Bluff
The summits of many bluffs are bare, rocky outcroppings. However, the harsh environment of bluffs are often vital ecosystems. Tiny organisms called lichen often colonize rocky bluffs, providing vital nutrients for many insects and birds.
Hardy grasses and shrubs can also take root on rocky bluffs. The shallow roots of these plants slow erosion of the bluff, even helping to secure valuable topsoil in some places.
Bluffs provide an ideal nesting spot for fishing birds such as cormorants and kingfishers. Cormorants build their nests on the bare ground of bluffs, bringing sticks and seaweed for construction. Cormorant colonies can grow so large that they take over the entire bluff. Kingfishers don’t nest on top the bluff, but dig a burrow directly in it. They prefer burrows on bluffs with little vegetation, as root systems get in the way of digging.
Bluffs are even home to endangered species. The El Segundo blue butterfly (Euphilotes battoides allyni), for example, is found only in a small bluff ecosystem near the giant Los Angeles International Airport in Southern California.