Weathering and erosion at Scotts Bluff: how heat, salt and ice break rock

A metal pipe that was level with the top of a bluff in 1933 now sticks out of the sandstone: the bluff, its caption says, has eroded approximately 18 inches in the past 75 years. Scotts Bluff National Monument is geologically active, and an NPS article explains how weathering and erosion are at work there. Rock slides are the visible results, though rarely does anyone see one happen. Less noticeable is water erosion from rain and snowmelt, and wind erosion that happens nearly every day.

Saddle Rock shows evidence of hundreds of years of weathering and erosion. Credit: NPS.

Two processes

• Weathering breaks rock down by heating and cooling, frost or crystal wedging, or chemicals such as acid in rain.
• Erosion removes the sediment, such as sand, silt or gravel, by wind or water. The products of weathering are carried off by erosion.

There are two types of weathering. Physical weathering comes from heating and cooling, the growth of foreign crystals and the collision of rock pieces during movement. Chemical weathering changes a rock or mineral after exposure to things like acid or oxygen: acid in rain can dissolve rocks such as limestone, and oxygen can make the iron often found in rocks corrode or rust.

How the bluff breaks

Rockfall like this is evidence of physical weathering, common in fall, winter and spring, when heating and cooling make rocks expand and contract. Credit: NPS.

The pipe was level with the top of the bluff in 1933. Credit: NPS.

Erosion: wind and water

The particles the bluffs shed are clay, silt and fine sand. Wind moves silt easily and over large distances, but moves sand only a few inches at a time as the grains roll. It removes material from one place and builds dunes elsewhere, from a few inches to hundreds of feet high; the sand hills of central Nebraska formed this way. Water, in runoff from summer storms or snowmelt, and in streams and rivers, is the more effective agent around Scotts Bluff and can move all particle sizes a long way. The badlands section of the monument shows how much.

Changing stream beds

Flowing water cuts channels that keep changing as the slope, the amount of water and the amount of sediment change. A channel adjusts its width, depth, number of channels and meanders, with erosion in some places and deposition in others, making cut banks and sandbars. Young channels are generally straight, with fast water, narrow valleys, boulders, gravel and coarse sand. Mature channels have long meanders, slower water, broad flat valleys and carry only smaller particles.

The North Platte River

Today's North Platte River is somewhere between young and mature: a wide, flat valley but no long meanders. Within the last 10,000 years it became overloaded with sediment and adjusted by forming braided channels, many small intersecting ones that move sediment more effectively than one large channel. A river that may once have been mature became a straighter, more youthful braided river within a mature valley.

Sources

• National Park Service, Scotts Bluff National Monument, "Weathering and Erosion" (https://www.nps.gov/articles/000/weathering-erosion.htm); the historic photograph credited to a private photographer is left out; rewritten in hubnx's own words.