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, while glacial deposits are the poorest sorted.

Step-by-step solution

Understanding Sediment Transport

First, recognize how each of the given modes of transport – stream, wind, and glacier – carries sediment. Streams transport through water movement, wind transports through air movement, and glaciers transport through ice movement.

Characteristics of Stream Deposits

Stream deposits often vary in grain size. Water can carry fine particles like clay and silt as well as larger particles like sand and gravel, though heavier particles settle faster. This can result in moderate sorting.

Characteristics of Wind Deposits

Wind deposits, such as dunes, tend to be well-sorted because wind carries only lighter particles. Coarser material generally stays behind, resulting in more uniform grain size, typically consisting of fine sand.

Characteristics of Glacier Deposits

Glacial deposits, called till, tend to be poorly sorted because glaciers carry a mix of all particle sizes: from boulders to clay. When glaciers melt, these deposits are left behind, with very little sorting.

Comparison and Conclusion

Based on the characteristics of each method of sediment transport: - Wind deposits will have the most uniform grain size due to excellent sorting. - Glacial deposits will show the poorest sorting, containing a wide variety of particle sizes.

Key concepts

Stream Deposits

Stream deposits are fascinating because they can tell us a lot about the movement of water in a particular area. Streams transport sediments using water flow, which can carry a range of different sized materials. You might find everything from tiny particles, like clay and silt, to larger grains like sand and even gravel in stream deposits.
In general, the energy of the flowing water determines the sizes of the particles that are carried. High-energy streams can transport larger particles, while low-energy streams tend to carry smaller ones. When the water slows down, heavier and larger particles settle first, while lighter particles carry on. This results in moderate sorting of the sediment. You can often see visible layers of different materials in stream deposits because of this natural sorting process.

Wind Deposits

Wind deposits, often found in deserts and sandy areas, consist primarily of finer particles like sand. Wind is a relatively weak force compared to water or ice. Consequently, it only transports lighter, finer materials while leaving behind larger, heavier particles. This makes wind deposits well-sorted, with a uniform grain size.
The sand dunes you see in deserts are an excellent example of wind deposits. Dunes are made almost entirely of sand because the wind isn't strong enough to carry larger rocks or pebbles. These fine sands are usually uniform in size and exhibit excellent sorting, making them unique compared to other forms of sediment deposits.

Glacial Deposits

Glacial deposits, also known as till, present a unique landscape differing greatly from wind and stream deposits. Glaciers are powerful, capable of picking up and transporting a mix of all sizes of sediments, from fine clay to massive boulders. However, unlike water and wind, glaciers do not sort their materials.
As glaciers move across land, they grind over bedrock, plucking stones and sediment along their path. When glaciers melt, all these mixtures of particle sizes are dumped together, resulting in poorly sorted deposits. You might find huge boulders right next to tiny grains of sand in glacial till. This lack of sorting showcases the immense power yet indiscriminate nature of glaciers as transporting agents.

Grain Size

The concept of grain size in sediments refers to the diameter of individual particles. Grain size can vary from microscopic clay particles to large boulders. Understanding grain size helps geologists determine the mode of transport and settling conditions.
In wind deposits, you will typically find all the grains to be similar in size, mainly fine sands. Stream deposits present more variation, allowing for a range of grains from tiny clays to larger gravels. Lastly, glacial deposits might offer a handful of all sizes, from microscopic to gigantic, reflecting the glacier's enormous transport capability across diverse terrains.

Sorting of Sediments

Sorting of sediments is a critical concept in geology, helping us understand how different deposits came to be formed. "Sorting" refers to the uniformity of particle sizes within a sediment deposit. Well-sorted deposits have particles of similar sizes, whereas poorly sorted deposits contain a mix of sizes.
Wind deposits are a textbook example of well-sorted sediments since they mostly comprise fine grains like sand. By contrast, stream deposits show moderate sorting due to varying water flow speeds, which transport different particle sizes. Glacial deposits, on the other hand, are the least sorted, with particles ranging from fine silt to enormous boulders, showcasing the glacier's indiscriminate collection ability.