Question

Describe how glacial erosion can create (a) a cirque, (b) a paternoster lake, and (c) striated bedrock.

Short answer

Glacial erosion creates cirques through plucking and abrasion, paternoster lakes by carving steps in valleys, and striated bedrock through debris scraping.

Step-by-step solution

Understanding Glacial Erosion

Glacial erosion is the process by which glaciers erode the landscape through various mechanical means, including plucking and abrasion. These processes lead to the formation of unique landforms, such as cirques, paternoster lakes, and striated bedrock.

How a Cirque is Formed

A cirque is a bowl-shaped depression found at the head of a glacial valley. It forms when a glacier erodes the rock beneath it through both plucking and abrasion. As the glacier moves, it picks up rocks (plucking) and rubs against the ground like sandpaper (abrasion), deepening the bowl shape over time. When the glacier retreats, it leaves behind this depression.

Formation of a Paternoster Lake

Paternoster lakes are a series of small, circular lakes formed along a glacial valley. They originate when a glacier carves steps in the bedrock as it moves, creating a valley with alternating high and low points. Once the glacier melts, water fills these depressions, forming a chain of lakes, much like beads on a string.

Creation of Striated Bedrock

Striated bedrock features lines or grooves cut into the rock surface by glaciers. These grooves are formed when rocks and debris embedded in the moving glacier scrape against the bedrock beneath it. The orientation of these striations can indicate the direction of glacier movement.

Key concepts

Cirque Formation

Cirques are stunning natural features typically found in mountainous regions. They are bowl-shaped, amphitheater-like hollows at the head of a glacial valley. The formation of a cirque is a classical result of glacial erosion and involves two primary processes: plucking and abrasion.

During the plucking process, a glacier picks up and carries away rocks and pieces of the bedrock as it moves. This is most effective on jointed or fractured rocks. Abrasion, on the other hand, involves the glacier's movement, which acts like sandpaper against the valley floor. Rocks and sediment embedded in the glacier's base grind against the bedrock, deepening and smoothing the depression. Over time, this dual action shapes the characteristic cirque form.

• The plucked rocks assist further erosion by acting as tools in abrasion.
• Quantitatively, the rate of erosion depends on the thickness and speed of the glacier.

When the glacier retreats due to melting, it leaves behind these distinctive cirques, often evolving into tranquil, circular tarns (small glacial lakes).

Paternoster Lakes

Paternoster lakes are a series of small, circular lakes that appear in succession down a glacial valley. The formation of these unique chains of lakes results from the glacier carving steps into the valley bed.

As a glacier moves, it erodes the underlying bedrock unevenly, creating areas of varying depth. These occur due to differences in the hardness of rock, with the glacier preferentially eroding softer areas, leaving behind a sequence of basins and sills. When the glacier eventually melts, these depressions fill with water, forming a string of lakes resembling beads on a rosary.

• The size and spacing of the lakes often depend on the original topography and the extent of glaciation.
• Paternoster lakes provide a visual story of a glacier's journey and influence on the landscape.

This pattern not only offers a beautiful sight but also a fascinating insight into past glacial activity.

Striated Bedrock

Striated bedrock is a prominent glacial feature characterized by long, linear grooves etched into rock surfaces. These striations are like nature's tell-tale signs of the power and dynamics of historic glaciers.

As glaciers advance, they entrain rocks and sediments at their base. These materials act like natural chisels and cut grooves into the bedrock over which the glacier travels. The resulting striations reveal the direction of glacial movement, appearing parallel to the glacier's flow.

• Striations can vary in width and depth, depending on the size of the embedded rocks and the force applied by the glacier.
• They serve as important geological clues for scientists studying historical glacial movements and past climate conditions.

So next time you see a boulder or rock with parallel lines, imagine the massive glacier that once passed over it, leaving its mark on the earth for aeons to come.