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 are most uniform in grain size; glacial deposits have the poorest sorting.

Step-by-step solution

Understanding the Sources of Sediment

Sediment can be deposited by different natural processes: streams, wind, and glaciers. Each process carries sediments of various sizes and compositions and deposits them differently, affecting the uniformity of the grain sizes and sorting quality.

Characteristics of Stream Deposits

Streams can carry a wide range of sediment sizes, including fine silt, sand, gravel, and even larger rocks. The flowing water sorts sediments based on their size and weight, often leading to relatively well-sorted deposits due to the carrying capacity of the stream's current, which can decrease in velocity over time, dropping heavier particles first.

Characteristics of Wind Deposits

Wind primarily transports and deposits fine particles such as sand and silt. The lighter and smaller particles are carried over longer distances, resulting in relatively well-sorted deposits, such as sand dunes or loess plains, where fine particles are fairly uniform in size due to the wind's selective transport ability.

Characteristics of Glacial Deposits

Glaciers transport mixed sediment sizes, including clay, silt, sand, gravel, and large boulders, as they move. As glaciers advance and retreat, they deposit this unsorted debris, called till. The diversity of grain sizes results in poorly sorted sediment because glaciers carry all sizes without sorting them by weight or size.

Evaluating Uniformity of Grain Size

When comparing uniformity in grain size, wind deposits are usually the most uniform due to the selective nature of wind transport, which can only carry and deposit similar grain sizes over a distance.

Evaluating Sorting Quality

Glacial deposits exhibit the poorest sorting because glaciers transport and deposit a wide variety of sediment sizes all at once, without the sorting action of water or wind. This results in a heterogeneous mix called till, with little distinction between material sizes.

Key concepts

Stream Deposits

Streams are nature's conveyor belts, transporting a variety of sediment sizes from one location to another. These water currents can carry fine particles like silt and sand, all the way up to larger particles such as gravel and rocks.
The key aspect of stream deposits is their ability to sort sediments based on size and weight. As the stream's velocity changes, it affects its carrying capacity. Faster currents can transport larger particles, but when the current slows down, the heavier particles get deposited first.
This process often leads to well-sorted sediment banks where sediments are grouped by size. Streams play an important role in shaping landscapes, creating features like riverbeds and deltas, which result from the accumulation of these sorted materials.

Wind Deposits

Wind deposition is a fascinating process where the movement of air transports and deposits fine particles like sand and silt over significant distances. This phenomenon is most evident in deserts and coastal environments, where features like sand dunes and loess plains showcase wind's power.
The lighter nature of these particles makes them perfect candidates for wind transportation. Because wind selectively carries smaller and lighter particles, the deposits that result are often well-sorted and consist largely of uniform grain sizes.
These deposits' homogeneity is due to the wind’s limited ability to transport heavier materials, ensuring the uniformity of sediment in regions influenced by large wind forces. This makes areas affected by wind deposition visually striking and crucial for understanding sedimentary processes.

Glacial Deposits

Glaciers are majestic but chaotic sculptors of the Earth's surface. Unlike streams and wind, glaciers transport a wide range of sediment sizes. As glaciers move and retreat, they dump this mixture of debris—including clay, silt, sand, gravel, and even massive boulders—in a process called deposition.
The result is a type of sediment called till, which is poorly sorted due to the glacier’s indiscriminate transport method. Glaciers deposit everything simultaneously, without sorting particles by size or weight.
This mixture of grain sizes leads to unsorted, heterogeneous landforms like moraines, where you can find mixtures varying from fine powders to giant rocks. Understanding glacial deposits is essential, especially in studying past climate changes and ice age cycles.

Grain Size Uniformity

Grain size uniformity refers to the degree to which sediment particles are of similar size. Among the different depositional agents, wind ranks highest in producing uniform grain sizes.
This is due to wind's ability to selectively carry only lighter, finer particles over distances, effectively sifting out varied grain sizes. When wind deposits material, the result is often a nearly uniform layer of sediment, such as what is seen in sand dunes of deserts.
Stream deposits also achieve a degree of grain uniformity, although not as pronounced as wind deposits, due to the water’s sorting action. In contrast, glacier-deposited till showcases low uniformity with its wide range of particle sizes.

Sorting Quality

Sorting quality describes how well sediment grains are separated based on size. Superior sorting means that particles of similar size are grouped together, typically seen in wind and stream deposits.
Wind deposits often have excellent sorting due to the wind's selective transportation of light and fine materials, leading to deposits that are uniform in grain size across extensive areas. Streams also achieve moderate sorting quality because flowing water naturally separates particles by size and weight.
Glacial deposits, on the other hand, are poorly sorted. Since glaciers carry an unsorted mix of various sediment sizes and deposit them indiscriminately, these materials tend to form heterogeneous collections with broad variations in grain sizes. This characteristic makes glacial deposits an important topic in geology for understanding earth's ancient environmental conditions.