1.3.2. Transportation

Transportation agencies can be combinations of gravity, flowing water, or air, and moving ice.

Transportation refers to the process by which soil and sediment are moved from one location to another. Various agents such as gravity, flowing water, air, and moving ice contribute to this process. Each of these agents works differently, affecting how particles are transported over distances. For example, overwriting materials can be weathered down into smaller particles and then carried away by these agents for deposition elsewhere.

Think of a river carrying sand and small rocks. As water flows faster, it can carry smaller particles further downstream, while larger rocks might only move short distances. This is similar to how a conveyor belt moves different sizes of boxes to different sections of a warehouse.

In water or air, the grains become sub-rounded or rounded, and the grain sizes get sorted so as to form poorly-graded deposits. In moving ice, grinding and crushing occur, size distribution becomes wider forming well-graded deposits.

When soil particles are transported by agents like water or air, they undergo physical changes. For example, as particles tumble through a river, they become rounded due to friction with other particles. This sorting process can create deposits where particles of similar sizes are grouped. In contrast, ice movement grinds down materials, creating a mix of sizes that forms a well-graded deposit, where you might find both small and large particles together.

Imagine how pebbles on a beach are round and smooth from being washed by ocean waves, while the rough stones just lifted from a mountain might still be jagged. The ocean's constant movement rounds out the pebbles while sorting them based on size.

In running water, soil can be transported in the form of suspended particles, or by rolling and sliding along the bottom. Coarser particles settle when a decrease in velocity occurs, whereas finer particles are deposited further downstream.

Running water transports soil in two main ways: either by keeping particles suspended in the water or by allowing them to roll and slide along the bottom of the riverbed. Coarser materials, like gravel, tend to settle quickly when the water slows down, while finer particles, like silt, can be carried much further downstream before settling. This creates a gradient where smaller, lighter materials are found further away from the source.

Think of a river carrying various sizes of rocks. Imagine it’s a race: the sand particles are light and zoom downstream, while the larger rocks are slower and settle first when the river slows down, making it like they have lost the race.

Wind can erode, transport and deposit fine-grained soils. Wind-blown soil is generally uniformly-graded.

Wind is another powerful agent of soil transportation, particularly for fine-grained materials like dust or silt. As the wind blows, it can pick up these tiny particles, carrying them over great distances. When these particles settle, they often form deposits that are uniform in size, known as uniformly-graded deposits. This is common in desert environments where fine sands are blown and redeposited.

Consider how a gust of wind can lift sandy particles from the beach and carry them into the air, creating dandelion-like clouds of sand that rain down uniformly on the beach a few yards away.

Gravity transports materials along slopes without causing much alteration.

Gravity plays a crucial role in the movement of soil, especially on slopes. Materials can move downhill in landslides or as debris flows, with little to no alteration of their original shape and size. This process relies on the pull of gravity to move particles downward, making it a fundamental aspect of erosion and sedimentation.

Think about how a stack of pancakes on a tilted plate can slide down the slope. The pancakes stay relatively intact as they tumble, just like soil and rocks move when pulled by gravity.

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