Question

Compare 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

Understanding Sediment Deposits by a Stream

Sediments deposited by a stream are typically sorted due to the flowing water. As the water velocity decreases, the heavier and larger particles settle first, followed by smaller particles. This process creates sediments that are relatively well-sorted, with grains of similar sizes grouped together.

Understanding Sediment Deposits by the Wind

Wind-blown sediments, such as those found in deserts, are usually composed of fine particles like sand and silt. The wind is selective, carrying only light particles while leaving heavier ones behind. Therefore, wind deposits are often well-sorted and consist of uniform grain sizes due to the consistent wind speed and direction.

Understanding Sediment Deposits by a Glacier

Glaciers transport and deposit sediments as they move and melt. Glacial deposits, known as till, are typically poorly sorted. Glaciers can carry a mix of particles of all sizes, from fine silt to large boulders, depositing them randomly as the ice melts and moves. This process results in a mixture of sediment sizes.

Analyzing Uniformity of Grain Size

Based on the characteristics of sediment deposition by streams, wind, and glaciers, wind deposits should have the most uniform grain size. Because wind selectively carries only similar-sized particles, it leads to the deposition of very uniform materials.

Analyzing Sorting Quality

The poorest sorting is found in glacial deposits. This is because glaciers carry and deposit a mixture of different grain sizes randomly, without the selecting and sorting action that defines wind or flowing water deposition.

Key concepts

Stream Deposition

Stream deposition occurs when flowing water slows down, typically in rivers and streams. The energy of the water directly affects the types of sediments that are carried and later deposited. As the velocity decreases, larger and heavier particles settle first, while finer particles may continue to be carried until the flow slows even further.
Stream deposition is a vital geological process that significantly impacts landscape formation. Over time, streams can carve valleys and create deposits like sandbars or riverbanks.

• Heavier particles like gravel settle first.
• Silt and clay are the last to settle.

Thus, sediments deposited by streams are usually well-sorted with similar-sized grains grouped together. The smaller particles form layers over the larger ones, reflecting the stream's decreasing speed.

Wind Deposition

Wind deposition primarily occurs in dry, arid regions, where sediment transport is dominated by the wind. This process is called aeolian deposition and involves the movement of fine particles like sand and silt. The wind's selective nature allows it to carry only lighter, finer particles over long distances.
As a result of this selective transport, wind deposits are often very well-sorted. Due to consistent wind speed and direction, the sediment that is deposited tends to be uniform in grain size, forming features such as sand dunes and loess plains.

• Particles like silt and fine sand are common in wind deposits.
• Wind deposits often create unique features in the landscape.

This uniformity in grain size comes from the wind's ability to only carry particles of similar size, leading to well-sorted deposits.

Glacier Deposition

Glacier deposition takes place as glaciers advance and retreat, transporting a wide variety of sediments. Unlike streams and wind, glaciers do not sort the materials they move. This results in what is known as glacial till—an unsorted mixture that includes everything from fine silt to large boulders.
Glacial deposits are characterized by their lack of sorting, resulting in layers of sediment with varying grain sizes. As the glacier melts, these unsorted materials are left behind, creating landforms like moraines and drumlins.

• Glacial till consists of a mix of particle sizes.
• These deposits form rugged, uneven landscapes.

The poor sorting of glacial deposits stems from the fact that glaciers carry materials irrespective of size, depositing them as they melt and move.

Grain Size Uniformity

Grain size uniformity refers to how similar the particles in a sediment deposit are in size. This is an important factor when comparing different types of sediment deposition.
Wind deposits typically have high grain size uniformity because they carry particles that are similar in size. Stream deposits may have good uniformity, although not as high as wind deposits due to the varying water velocities affecting particle settlement.

• High uniformity: wind deposits.
• Moderate uniformity: stream deposits.

In contrast, glacial deposits have low grain size uniformity. The wide range of particles, from tiny silt to enormous boulders, means the grains vary greatly in size.

Sorting of Sediments

Sorting of sediments is a crucial process that describes how sediment grains are organized by size. It is closely tied to the energy and transport medium of the sediments.
Well-sorted sediments are comprised of grains of a similar size. This is often observed in wind and stream deposits. Wind, with consistent speed, results in very well-sorted sediment, while streams also demonstrate sorting but over a range of grain sizes depending on the changing speed of the water.

• Well-sorted: wind and stream deposits.
• Poorly sorted: glacial deposits.

Glacial deposits, on the other hand, are poorly sorted. Glaciers carry different sizes randomly and leave them in the same manner, leading to a chaotic mix of sizes. This sorting—or lack thereof—can tell us much about the depositional environment and the past activity of the geological processes.