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, while glacial deposits exhibit the poorest sorting.

Step-by-step solution

Sediment Deposited by a Stream

Stream deposits, known as alluvium, are typically well-sorted. Water selectively transports smaller and lighter particles further downstream, while larger sediments are deposited closer to the river's course. Due to the continuous flow, these deposits are often layered and sorted based on size, with finer materials like silt and sand dominating.

Sediment Deposited by the Wind

Wind deposits, called aeolian deposits, often consist of fine particles such as sand and silt. Because wind is a selective transporter that can only move small sediments for great distances, these deposits exhibit better sorting compared to glacial deposits. Deserts often display these uniform sand dunes.

Sediment Deposited by a Glacier

Glacial deposits are unsorted and heterogeneous, consisting of various grain sizes ranging from fine silt to large boulders. As glaciers move, they pluck and carry a wide array of sediments and deposit them en masse when the glacier retreats, resulting in poorly sorted tills.

Comparing Grain Size Uniformity

The wind is the most effective in sorting sediments by size due to its limited carrying capacity. Therefore, aeolian deposits have the most uniform grain size, primarily consisting of similarly-sized particles such as sand.

Determining Poor Sorting

Glaciers exhibit the poorest sorting of deposits. As glaciers transport materials of varying sizes together and deposit them simultaneously, they create unsorted collections of sediments, known as till.

Key concepts

Alluvial Deposits

Alluvial deposits are formed by streams and rivers as they flow over land. These water bodies play a significant role in the transportation and deposition of sediments. As streams travel downstream, they carry particles with them. However, not all particles travel the same distance or at the same speed.

• Heavier particles like gravel and pebbles settle quickly and are often found in places with faster-moving water.
• Finer particles such as silt and clay can be carried much further, depositing in slower-moving areas.

This sorting process causes alluvial deposits to be layered and relatively uniform in composition in specific areas. They showcase a range of grain sizes that align with the water flow's ability to transport materials efficiently.

Aeolian Processes

Aeolian processes refer to the influence of wind on earth's surface, particularly in the deposition and erosion of sediments. Wind is typically capable of transporting only smaller particles, like sand and silt, because it lacks the strength to move larger materials. This makes aeolian deposits unique.

• Sand dunes found in deserts are a classic example of aeolian deposits. Here, wind has deposited similarly sized particles in extensive uniform patterns.
• The directional nature of wind transport leads to well-sorted sediments.

This means that aeolian deposits often have the most uniform grain sizes, as seen in sandy environments.

Glacial Till

Glacial till is the sediment left behind as glaciers move and melt. Unlike wind and water, glaciers are capable of carrying a variety of particle sizes.
From fine silt to massive boulders, glaciers pick up material from the landscape and transport it to new locations.

• As glaciers advance and retreat, they deposit these diverse sediments in bulk, leading to unsorted accumulations known as till.
• This poor sorting is attributed to the non-selective nature of glacial transport.

The mixture of grain sizes in glacial till makes it distinguishable from other sediment deposits.

Grain Size Uniformity

Grain size uniformity refers to the consistency of particle sizes within a sediment deposit. Aeolian deposits, being primarily influenced by wind, demonstrate high grain size uniformity.

• Only smaller particles are transported over large areas, creating sandy deposits of uniform size.
• This consistency is seen in areas like dunes, where sand grains are closely similar in size.

In contrast, river and glacial deposits may display variability in grain sizes, due to the differences in transporting mechanisms and energy levels.

Sediment Sorting

Sediment sorting is the process by which particles are separated and organized based on their size, shape, and density. Better sorting is often achieved by processes that have a selective transport mechanism, like wind.

• Alluvial deposits from streams tend to be moderately well sorted. The water's current sifts the particles, depositing similar-sized grains together.
• In aeolian environments, the wind's capacity to carry only smaller particles means sediment sorting is quite refined.
• Glacial till exhibits the poorest sorting due to glaciers' ability to transport all sizes of particles without discrimination.

Understanding sorting helps geologists and students comprehend the history of sedimentary environments and the dynamics of past depositional processes.