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 large deposit of sediment 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 likely possibility, you remind your companion that material deposited by landslides can also consist of an unsorted mix of many sizes. How might you and your friend determine whether this deposit is actually glacial till?

Short answer

Examine sediment size, particle shape, surrounding topography, striations, and deposit structure for glacial till indicators.

Step-by-step solution

Observe the Sediment Size and Composition

Begin by examining the sediment to see if it is poorly sorted, meaning it contains particles of various sizes from clay to boulders. Glacial till often has a distinctive unsorted mix, while landslide deposits can vary but might be slightly more sorted due to gravity sorting during movement.

Examine the Shape of Particles

Look closely at the shape of the sediment particles. Glacial till tends to have more angular particles, as the ice does not round them significantly, unlike landslide deposits, which might have more rounded particles due to tumbling.

Consider the Surrounding Topography

Assess the surrounding area for signs of glacial activity, such as moraines and U-shaped valleys, which are indicators of past glacial movement. If such features are present, it strengthens the argument for glacial till.

Check for Glacial Striations

Look for striations or scratches on larger rocks within the deposit. These are often caused by rocks embedded in ice scraping against each other and bedrock, which is indicative of glacial activity.

Consider the Overall Deposit Structure

Glacial tills can form mounds or ridges known as drumlins or moraines, while landslide deposits are more likely to form chaotic heaps or runouts. Compare the deposit’s structure with these characteristics.

Key concepts

Sediment Size and Composition

When identifying glacial till, the sediment's size and composition are crucial clues. Glacial till is recognized by its poor sorting, meaning the mix contains a wide range of sediment sizes, from tiny clay particles to large boulders. This variety occurs because glaciers transport everything in their path without discrimination.
Landslides, while also producing mixed-size deposits, often show slight sorting due to the tumbling action of gravity, which can separate finer particles from coarser ones to some extent. Therefore, when analyzing a deposit, make sure to check whether there's a broad range of sizes present. If so, it's a strong indication of glacial activity.
Understanding this fundamental aspect is essential when differentiating between deposits caused by glaciers and those by landslides.

Particle Shape Examination

To help determine if a deposit is glacial till, examining the shape of individual sediment particles is invaluable. Glacial till often contains more angular particles. The reason for this is simple: as a glacier moves, it picks up and transports rocks, which rarely have prolonged contact with each other.
This results in sediments that are not subjected to the same degree of rounding or smoothing. By contrast, landslide deposits exhibit particles that more often have rounded edges, a result of particles tumbling and grinding against each other as they move downhill. Checking for the angularity of sediments can offer telltale signs of glacial origin.

Glacial Topography Indicators

The surrounding landscape can provide significant evidence when trying to identify glacial till. Glaciers dramatically shape landscapes, creating distinct features in the topography. Look for classic glacial formations such as moraines, which are accumulations of glacial debris, and U-shaped valleys, which result from the glacier's path.
These features are indicators of previous glacial movement and support the likelihood that a nearby sediment deposit was caused by glacial, not landslide, activity. Observing these indicators can significantly aid in confirming the origin of a sediment deposit.

Glacial Striations

Glacial striations are some of the most compelling evidence of glacial activity. Striations are scratches or grooves carved into the surface of larger rocks by other rocks or particles carried by glaciers.
By examining the larger stones within a deposit for such markings, one can often confirm the presence of glacial action. These striations occur because as glaciers move, rocks embedded in the ice scrape against other rocks and surfaces, leaving behind distinct linear marks. Finding these on rocks within a sediment deposit strongly suggests that the deposit is indeed glacial till.

Deposit Structure Analysis

Analyzing the overall structure of a sediment deposit can provide crucial insights. Glacial till commonly forms specific structures like drumlins, elongated hills, or moraines, which are accumulations of debris. These formations differ significantly from those produced by landslides.
Landslide deposits typically present more as chaotic heaps or runouts, where material has simply fallen into place. Observing whether the sediment deposit forms organized structures, like mounds or ridges, as opposed to a more haphazard arrangement, can help determine its origin. Thus, examining the deposit's structure is essential in identifying it as glacial till.