Sedimentary Environment

Water, wind, and ice erode, transport, and deposit terrigenous sediments on land. Geologists recognize five common terrestrial sedimentary environments: stream, lake, desert, glacial and volcanic.

Streams are the most widespread terrestrial sedimentary environment. In fact, because they dominate landscapes in both humid and arid climates, stream valleys are the most common landform on Earth. Streams naturally meander and coarse-grained sediments accumulate along the inside of meanders where water velocity decreases, forming sand and gravel bars. When flood waters overflow a stream's banks, fine-grained sediment accumulates on the land surface, or floodplain, adjacent to the channel. Coarser sediment collects on the channel banks during floods, forming a narrow deposit called a levee. Sorting, rounding, and sediment load generally increase downstream.

Where a stream rapidly changes from a high to low slope on land, for example at the base of a mountain, gravel, sand, silt and clay form a sediment pile called an alluvial fan. Where a stream flows into standing water its sediments produce a deposit called a delta. Deltas are usually finer grained than alluvial fans. In both alluvial fans and deltas, grain size rapidly decreases downslope.

Most lakes form from water contributed by one or more streams as well as precipitation directly into the lake. As it arrives at a lake, stream velocity drops very rapidly, depositing the coarsest sediment at the lakeshore and forming a delta. Farther from shore, as the water continues to lose velocity, finer and finer grained sediment falls to the lake bottom. Only in the deepest part of the lake is water movement slow enough to permit the finest grained sediment to accumulate. This produces thin layers of clay. Hence, grain size generally decreases from the lakeshore to its center.

Deserts develop where rainfall is too sparse to support abundant plants. Contrary to popular belief, deserts are not typically vast seas of sand. Instead, they consist mostly of a mixture of gravel and sand. However, the sand may be eroded away, or deflated, by the wind leaving behind a layer of gravel called a desert pavement, or reg. The deflated sand is later heaped into piles downwind, producing dunes. In spite of the prevalence of regs and dunes in deserts, water is nonetheless the most important agent of erosion. Alluvial fans are common at the base of mountains. Dry lake beds, or playas, and salt deposits, or sabkhas, resulting from lake evaporation, commonly occupy the adjacent valley floor.

Where snowfall exceeds snowmelt, ice accumulation eventually forms a glacier. Alpine glaciers occur throughout the world on mountains at high elevations. Modern continental glaciers now cover Antarctica and Greenland. From around two million years ago to about ten thousand years ago—the Pleistocene epoch, or "Ice Age"—glaciers deposited sediments over large areas at mid- to high-latitudes. These glacial ice deposits, called till, are characterized by very poor sediment sorting. They generally are thick, widespread sheets or narrow, sinuous ridges. Ice meltwater forms thick, widespread layers of sediment, called stratified drift, with good sorting.

Though volcanism involves igneous processes, volcanic sediments compose much of terrestrial volcanic deposits. These volcaniclastic, or pyroclastic, sediments form when ash, cinders, and larger volcanic materials fall to the ground during eruptions. Running water often modifies volcaniclastic sediments after deposition. They also may move downhill as a mudflow, or lahar, when saturated with water. Generally, volcaniclastic sediments form thin lobe-shaped deposits and widespread sheets, which thicken toward the volcanic source.