Question

Compare and contrast the sediment deposited by a stream, the wind, and a glacier. Which deposit should have the most uniform grain size? Which one would exhibit the poorest sorting? Explain your choices.

Short answer

Wind deposits have the most uniform grain size. Glacial deposits exhibit the poorest sorting.

Step-by-step solution

Understand the Depositional Processes

Streams, wind, and glaciers are agents of sediment transport and deposition. Each agent affects the size and sorting of the sediment differently. Recognize that streams usually deposit sediments of varying sizes, wind typically deposits finer grains, and glaciers leave sediments of various sizes unsorted.

Stream Deposits

Stream deposition results from water flow. As water slows down, it deposits sediments starting with larger particles first and smaller ones later, leading to graded bedding. Streams usually create deposits with varying grain sizes based on the velocity of the water.

Wind Deposits

Wind, having less energy, mainly transports and deposits smaller particles like sand and dust. The sediment deposited by wind is typically well-sorted, consisting mostly of similarly sized grains, particularly in dunes.

Glacier Deposits

Glaciers transport all sediment sizes, dragging them from erosion zones to deposition zones. When glaciers melt, they leave behind till—an unsorted mix of clay, sand, gravel, and boulders. This deposit has variable grain size.

Compare Uniform Grain Sizes

Identify which agent's sediment is most uniform in grain size. The wind is most effective at sorting sediments, resulting in the most uniform grain sizes among these agents, because it mainly transports similar-sized small particles.

Identify Poorest Sorting

Determine which agent's sediment exhibits the poorest sorting. Glacial deposits have the poorest sorting because they contain a mix of all different grain sizes, from clay to large boulders.

Key concepts

Stream Deposits

Streams are dynamic forces of nature, constantly moving and shaping the landscape by transporting sediment. As the velocity of a stream changes, it affects the sediment it carries. When a stream slows down, larger materials such as rocks and gravel are deposited first. This process continues with the deposition of progressively smaller particles like sand and silt.

Thus, stream deposits often show a graded bedding, wherein there is a layered arrangement of material. Near the source of a stream or river, fast-moving water may carry and deposit larger rocks. Further downstream, finer sediments are more commonly found as the stream loses energy. However, what is unique about stream deposits is their variability in grain sizes due to fluctuations in water speed and volume.

• Characteristics: Varied grain size
• Examples: Graded bedding, floodplains
• Sorting: Moderate sorting

Wind Deposits

Wind, though seemingly gentle, is an incredibly effective sorting agent for sediment. Because wind has less energy compared to water or ice, it primarily transports smaller particles like sand, fine dust, and small pebbles. As a result, wind deposits are well-sorted and primarily consist of uniform grain sizes.

When wind speed decreases, it deposits sediments in characteristic formations such as sand dunes. These dunes are typically made of fine to medium sand grains and may form in various shapes based on wind direction and velocity changes. The ability of wind to separate fine particles from coarser ones makes it a natural sorting machine.

• Characteristics: Small, uniform grain sizes
• Examples: Sand dunes, loess deposits
• Sorting: Good sorting

Glacier Deposits

Glaciers are massive, slow-moving bodies of ice that can transport a wide range of sediment sizes as they advance and retreat. Unlike wind or water, glaciers have the strength to carry large chunks of rock as well as finer particles. As glaciers move, they scrape up all kinds of material, which is dumped in a somewhat chaotic fashion when the glacier melts.

This unsorted mix of sediment is known as "till," and it can consist of everything from fine clay to massive boulders, often piled into features like moraines. Consequently, glacier deposits exhibit extremely poor sorting, since no natural process within the glacier separates large particles from small ones.

• Characteristics: Mixed, unsorted grain sizes
• Examples: Till, moraines
• Sorting: Poor sorting