Question

While taking a break from a hike in the Northern Rockies with a fellow geology enthusiast, you notice that the boulder you are sitting on is part of a deposit that consists of a jumbled mixture of many different sediment sizes. Since you are in an area that once had extensive valley glaciers, your colleague suggests that the deposit must be glacial till. Although you know this is certainly a good possibility, you remind your companion that other processes in mountain areas also produce unsorted deposits. What might such a process be? How might you and your friend determine whether this deposit is actually glacial till?

Short answer

Another process could be a landslide. Examine the sediments; rounded and polished features suggest glacial till.

Step-by-step solution

Understanding the Scenario

The problem involves identifying a deposit of mixed sediment sizes found in a mountainous area previously covered by glaciers. The goal is to determine whether this deposit is glacial till.

Identifying Possibilities of Formation

In mountainous environments where glaciers existed, deposits can be formed by glacial till or other processes. One such process is a landslide, which can also lead to unsorted sediment deposits.

Distinguishing Processes

While both glacial till and landslides can create mixed sediment deposits, glacial till contains sediments that are typically scratched, rounded, and polished by glacier movement. Landslides, however, result in more angular and less weathered particles.

Analyzing Sediment Characteristics

Examine the sediment characteristics. If the sediments are more angular and not rounded or polished, this might indicate a landslide process. In contrast, if the sediments exhibit traits like scratching, rounding, and polishing, glacial till is more likely.

Conducting a Field Test

To determine the deposit type, collect samples and examine them under a microscope or use hand lenses to look for specific features like striations or polish typical of glacially transported sediments.

Key concepts

Sediment Deposits

Sediment deposits appear in various forms and sizes depending on the geological processes involved. These deposits consist of a range of particle sizes which can include anything from very fine silt to large boulders.
In the case of glacial till, this sediment type is particularly fascinating because it reflects the sheer, mixed nature of the material left behind by retreating glaciers. The characteristics of these deposits often include:

• Unsorted, which means the sediment sizes vary greatly from fines to large stones.
• Lack of stratification or layers, since they are directly dumped by melting glaciers.
• Presence of scratched or polished stones, evidencing movement across other rocks.

When studying sediment deposits, one key is looking for uniform patterns or a complete lack of them, as unsorted deposits could imply either glacial or other geological processes like landslides.

Valley Glaciers

Valley glaciers, also known as alpine glaciers, are essentially rivers of ice flowing down mountain valleys. They play a significant role in shaping the landscape by eroding and transporting sediments. These glaciers form in high mountains where temperature and conditions allow snow and ice to accumulate over time.
Key features of valley glaciers include:

• Confined pathways which follow existing valleys.
• Significant erosion capabilities, altering the landscape by carving out deep troughs.
• Tendency to deposit glacial till, a mix of sediments, when they retreat.

As these glaciers advance and retreat, they grind the rock below, leading to the creation of mixed sediment deposits. The material they leave behind, such as glacial till, serves as evidence of their past activity, with characteristics like rounded stones due to prolonged contact with moving ice.

Geological Processes

Various geological processes contribute to the formation of mixed sediment deposits in mountainous regions. Among these processes, glacial movement and landslides are two of the most prominent, each leaving telltale signs in their deposits.
Unlike the scratching and polishing in glacially transported sediments, landslides produce deposits that can appear chaotic without sorted layers. This unsorted nature is due to the sudden downhill movement of rock and soil.
Important differences include:

• Angular sediments pointing to a landslide, as opposed to rounded ones from glacier action.
• A lack of significant surface abrasion in landslide deposits, unlike glacial till residues.

Understanding the nuances between these processes allows geologists to determine the origins of sediment deposits and thereby unravel the geological history of the area. This analysis includes examining sediment under microscopes to pick out distinct features indicative of the responsible process.