Question

Compare and contrast the sediment deposited by a stream, 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, and glacial deposits exhibit the poorest sorting.

Step-by-step solution

Understanding Stream Deposits

Stream deposits, or alluvium, usually consist of sand, silt, clay, and gravel transported by flowing water. Streams sort materials based on particle size, with larger particles settling first due to gravitational pull and water flow velocity. As a result, stream deposits are typically well sorted, especially in areas where streams slow down, like on riverbanks or floodplains.

Exploring Wind Deposits

Wind deposits, such as dunes and loess, are formed by the wind's ability to carry and deposit fine particles like sand and silt. Wind sorting is very effective, typically leading to deposits with well-sorted and uniform grain sizes, as wind can only transport finer materials over long distances.

Investigating Glacial Deposits

Glacial deposits result from the melting and movement of glaciers, which pick up and transport a wide range of particle sizes, from fine silt to large boulders. These are called till, and they are generally very poorly sorted as glaciers indiscriminately drop a mix of everything they transport when they melt.

Comparing and Contrasting Deposits

Stream deposits tend to be well-sorted, particularly in areas of slower water flow, due to the sorting action of water. Wind deposits are typically the most uniform and well-sorted because of the wind's capacity to transport only finer particles, creating even-sized grain deposits. Glacial deposits are the poorest sorted due to the randomness of the debris left behind by glacier movement.

Key concepts

Stream Deposits

Stream deposits are fascinating geological features resulting from the action of flowing water. As streams move, they carry particles like sand, silt, clay, and gravel. The sorting process depends significantly on water velocity and particle size.

• In faster-moving parts of the stream, larger particles like gravel settle first due to gravitational pull.
• As streams slow down—perhaps on riverbanks or floodplains—finer particles like silt and clay begin to settle.

This natural filtering process leads to stream deposits being generally well sorted. However, the extent of sorting can vary based on how a stream meanders and the terrain it flows over. Understanding stream deposits helps geologists deduce the speed and history of ancient water flows.

Wind Deposits

Wind deposits highlight another form of natural sorting driven by the power of air movement. These deposits often include features like sand dunes and loess, formed due to wind's ability to transport fine, light particles over long distances. A key feature of wind deposits is:

• Efficiency in sorting: Wind primarily carries smaller particles such as sand and silt, which results in deposits that are well sorted and quite uniform in grain size.

This uniformity occurs because larger particles are not easily lifted by the wind, thus remaining behind. These deposits create unique landscapes that can be extensively studied to understand wind patterns and climates.

Glacial Deposits

Glacial deposits, known as till, result from the gradual melting and movement of glaciers. As glaciers traverse landscapes, they accumulate and transport a variety of particle sizes. Unlike stream or wind deposits, glacial deposits have less uniformity due to the sheer diversity of particles they carry. Glaciers can pick up anything from fine silt to massive boulders.

• The randomness in particle size is attributed to the glacier's ability to transport a mix of all sediment types without selective sorting.

As a glacier melts, it unceremoniously drops its mixed-load, thus leading to poorly sorted deposits. Studying glacial deposits offers insights into past glacial movements and the geography they carved out.

Grain Size Uniformity

Grain size uniformity in sediment deposits varies substantially based on the transportation agent—water, wind, or ice. This characteristic is crucial for understanding the depositional environment and the processes involved. In general:

• Wind deposits have the most uniform grain size due to wind's ability to move only finer particles over long distances.
• Stream deposits follow, with sorting largely influenced by water velocity that filters sediment progressively.
• Glacial deposits exhibit the least uniformity, as glaciers carry a wide range of particle sizes from silt to boulder without preference.

Understanding these differences helps predict the nature of deposits in different geological settings, which is important for fields like sedimentary geology and soil science.

Sorting Efficiency

Sorting efficiency describes how effectively a geological agent separates particles based on size. This characteristic can tell us a lot about the depositional environment.

• Wind deposits are the most efficiently sorted because wind consistently moves only finer sediments like sand and silt.
• Stream deposits are reasonably well sorted, as water velocity influences which particles are deposited and when.
• Glacial deposits are the least sorted due to the random mixture of sediment sizes transported by glaciers.

Recognizing sorting efficiency helps geoscientists make inferences about past environmental conditions and it significantly aids in reconstructing Earth's geological history.