What Is Inverse Bedding?

Weathering Processes

• Most surface soil areas originate from large rock formations found deep beneath the ground’s surface. Over time, water seeps through ground layers and breaks down larger rock forms as wind blows small surface particles to new locations. As rocks become smaller, they move upward through the ground’s layers. Under normal conditions, the smaller the rock particle the farther up it moves in the ground layer sequence. When extreme or unusual weathering processes occur, rock and particle layers may form inclines or large rocks may end up in the top layers. Inverse bedding exists in areas where large rock particles occupy the top layers of the ground and small particles or pieces rest in the bottom layers.

Graded Bedding

• Graded bedding consists of different soil types, such as rock, sediment, clay and compost, that appear in layers. Each layer results from a different type of weathering process that occurs over a period of time. Weathering processes involve mechanical forces, such as wind and water, or chemical forces, such as climate effects and plant interactions with the soil. Inverse bedding is a type of graded bedding that develops under unusual conditions. For example, large influxes of water can reverse normal bedding layers and cause large rock particle layers to rest on top of small particle layers. Inverse soil layers also can result from ground excavations performed during the course of a construction project.

Particle Deposits

• The soil layers present within an inverse bedding pattern result from erosion forces -- such as strong winds or flowing waters -- that move larger rock particles at a faster rate than normal. The force of the wind or water determines what size particles erode from their original points. Over time, weather patterns create beds, or layers of soil made up of different rock and soil particle sizes. Weather events involving large influxes of water, such as hurricanes, can cause abrupt upheavals within soil layers and reverse the natural order of particle sizes within a soil bed.

Conditions

• Natural graded soil beds develop from gradual conditions that move rock and soil particles according to size, shape and density. As small soil particles tend to weigh less than the larger sediment pieces, the larger pieces tend to settle at deeper layers than the particles. Flat or angular sediments also tend to settle at slower rates than round sediments. In cases where extreme wind or water flow enters a soil bed, these conditions tend to reverse. In effect, the impact of wind or water peels away the surface layers of the soil, transports them to a new area and piles the larger sediment layers on top.