Question

If the budget of a valley glacier were balanced for an extended time span, what feature would you expect to find at the terminus of the glacier? Is it composed of till or stratified drift? Now assume that the glacier's budget changes so that ablation exceeds accumulation. How would the terminus of the glacier change? Describe the deposit you would expect to form under these conditions.

Short answer

A balanced glacier forms till at its terminus, while retreating glaciers deposit both till and stratified drift.

Step-by-step solution

Understanding Glacier Terminology

A glacier "budget" refers to the balance between ice accumulation and ablation (melting and sublimation). The terminus of the glacier is the end of the glacier.

Balanced Glacier Budget

If the glacier's budget is balanced, the glacier's terminus remains stationary because ice accumulation equals ice loss. At this point, you would typically find a terminal moraine, composed of unsorted debris called till, deposited by the glacier as it moves.

Imbalanced Glacier Budget

If ablation (ice melt) exceeds accumulation, the glacier will retreat. The terminus of the glacier would move back, leaving behind glacial deposits.

Features of Glacial Retreat

As the glacier retreats, it leaves behind moraines and other deposits such as stratified drift, which are sorted by meltwater and consist of layers of sediment.

Describing the New Deposits

Under these conditions, expect to find a mix of till and stratified drift. The stratified drift forms due to meltwater sorting and depositing sediments downstream.

Key concepts

Glacier Terminus

The glacier terminus, also known as the snout or toe, is the endpoint or farthest extent of a glacier. It's an important feature because it helps scientists track changes in a glacier's size over time. The position of the glacier terminus can move forward, backward, or stay the same, depending on the glacier's budget. The glacier budget refers to the balance between ice accumulation at the glacier's source and the ablation, which includes melting and sublimation. When the glacier budget is balanced, meaning accumulation equals ablation, the terminus remains stationary. However, if ablation exceeds accumulation, the glacier retreats and the terminus moves back.

Till

Till is an unsorted mixture of sediments deposited directly by a glacier as it moves over the land. It consists of a wide range of materials, from fine silt to large boulders. Till is left behind as a terminal moraine when the glacier's terminus remains stationary for a long period, indicating a stable glacier budget. These moraines are found at the leading edge of the glacier and are a record of the glacier's past positions. Till is distinct for being unsorted and unstratified, which means the individual rocks and particles are mixed together without layers, reflecting the direct deposition by the ice.

Stratified Drift

Stratified drift refers to sediments transported and deposited by meltwater from the glacier. Unlike till, these deposits are sorted and layered due to the action of flowing water. As glaciers melt, water flows across the landscape, picking up and carrying sediments downstream. The heavier particles settle out first, followed by finer materials, creating distinct layers. Stratified drift is more organized than till and often found in outwash plains and other areas where meltwater has carried the sediment away from the glacier. These deposits provide clues about past glacial meltwater flows and the sedimentary processes associated with glacial environments.

Glacial Retreat

Glacial retreat occurs when the glacier loses more ice to melting and sublimation than it gains from snow accumulation. This imbalance in the glacier's budget causes the terminus to pull back from its previous position. As glaciers retreat, they leave behind various geological features. From piles of till, forming moraines, to stratified drift sorted by meltwater. These features not only help geologists study past climates and environments but also indicate changes in glacier dynamics. When glacial retreat is observed, the landscape alters significantly, showcasing the powerful force of glacial ice and meltwater in shaping the terrain.