Question

Why do glacial deposits show poor sorting?

Short answer

Glacial deposits are poorly sorted because glaciers carry and deposit a wide range of sediment sizes without sorting them.

Step-by-step solution

Understanding Sorting in Geology

Sorting is a geologic term used to describe the distribution of grain sizes within a sediment or rock. Well-sorted sediment contains grains that are almost all the same size, while poorly sorted sediment contains a wide range of grain sizes.

Defining Glacial Deposits

Glacial deposits are sediments that are left behind after glaciers melt. These deposits can include a variety of materials, ranging in size from fine silt and clay to large boulders.

Analyzing Glacier Movement and Deposition

As glaciers move, they carry with them all types and sizes of materials, picked up by the glacier through abrasion and plucking. This transport process does not discriminate based on size, leading to a mix of different grain sizes.

The Deposition Process

When glaciers melt, they release the sediment they carry. This melting is a rapid process that doesn't allow for sorting of materials by size, resulting in a poorly sorted mix of fine and coarse particles.

Conclusion: Why Glacial Deposits Have Poor Sorting

The poor sorting found in glacial deposits is due to the non-selective process by which glaciers collect and deposit sediment of various sizes, combined with rapid melting that doesn't facilitate sorting by weight or size.

Key concepts

Sorting in Geology

Sorting in geology refers to the distribution of sediment grain sizes. When we look at well-sorted materials, every grain tends to be similar in size. Imagine a jar full of equally sized marbles. Poorly sorted materials are quite different. They contain a variety of grain sizes, somewhat like a jar filled with marbles, sand, and boulders all mixed together.
In sedimentology, sorting gives us clues about the history and processes that formed the sediment. Wind, water, and ice can all sort materials differently, based on their movement ability and the time they take to settle.
Sorting helps us understand past environments or conditions in which a sediment or rock was deposited. For example, wind-blown deposits are usually well-sorted because wind can only carry small grains over large distances. Glaciers, however, exhibit poor sorting due to their unique transport methods.

Glacier Movement and Deposition

Glaciers are like slow-moving rivers of ice, constantly transporting materials as they slide over the land. This movement happens due to gravity, pushing glaciers downhill. As they move, glaciers pick up a wide range of debris in a process known as abrasion and plucking.
As the glacier advances, it grinds down bedrock, pulling loose particles and even larger rocks into its icy body. This debris, known as glacial till, is transported along with the glacier. Since glaciers gather materials of all grain sizes, from sand to large boulders, the resulting deposits have varied sizes.
When a glacier eventually melts, it releases all these materials in a jumbled mix. Therefore, the manner in which glaciers move and deposit their load doesn't allow for sorting during deposition, leading to a chaotic assortment of different materials.

Sediment Grain Sizes

Sediment grain sizes refer to the physical dimensions of particles within a sediment or sedimentary rock. These sizes can range from tiny clay particles less than 0.002 mm in diameter to large boulders greater than 256 mm.
The classification of sediment by grain size is important because it provides information about the environment in which the sediment was formed or deposited. Finer grains like clay and silt often indicate a low-energy environment such as a lake, while larger grains like gravel and boulders may suggest high-energy conditions such as a swift-flowing river or a glacial environment.
Understanding sediment grain sizes also gives insights into the transport and deposition process, as different forces—such as wind, water, and ice—can carry particles of differing sizes over varying distances.

Glacial Sediment Transport

Glacial sediment transport is unique compared to other geological processes because it involves the non-discriminatory movement of a wide range of particles. As glaciers move, they transport a mixture of sediments, known as glacial till, which can include everything from fine dust to massive boulders.
This mixture happens because glaciers flow due to gravitational pull, dragging sediments along. The force of this movement grinds some materials and collects others, embedding particles of all sizes within the glacier or at its base.
Unlike water or wind, which can sort particles during transport (smaller particles travel further), glaciers do not have this sorting mechanism. When a glacier melts, it drops its sediment all at once, leading to poorly sorted deposits. This lack of sorting during transport and deposition is why glacial sediments are so diverse in particle size.