Glacial Meltwater Landforms: More than Just Ice!

When we think of glaciers, we usually picture massive, slow-moving walls of ice. However, water plays a huge role in shaping the landscape. As glaciers melt—especially during warmer periods or in the summer—they produce massive amounts of meltwater. This water is incredibly powerful because it often flows under high pressure, carrying huge loads of debris.

In this chapter, we explore how meltwater creates unique landforms. We call these fluvioglacial landforms ("fluivio" for river/water and "glacial" for ice). Don't worry if the names sound a bit strange at first; we will break them down step-by-step!

1. Glacial vs. Fluvioglacial: What’s the Difference?

Before we look at the landforms, it is important to understand how water acts differently than ice. This is a common area where students lose marks in exams!

  • Glacial Deposits (Till): These are dropped directly by the ice. The rocks are unsorted (all sizes mixed together) and unstratified (no layers). The stones are often angular because they haven't been smoothed by water.
  • Fluvioglacial Deposits (Outwash): These are dropped by meltwater. They are sorted by size (the water drops the heaviest rocks first) and stratified (laid down in clear layers or "strata"). The stones are usually rounded because they have bumped into each other in the flowing water.

Quick Tip: If you see a photo of sediment in an exam, look for layers. If there are layers, it’s likely a meltwater landform!

2. Meltwater Erosion: Meltwater Channels

Meltwater doesn't just flow on top of the ice (supraglacial); it also flows inside the ice (englacial) and underneath it (subglacial). Because this water is often trapped under the weight of the ice, it is under hydrostatic pressure. This means it flows very fast and can erode deep channels into the solid bedrock.

Meltwater Channels (sometimes called "overflow channels") are deep, steep-sided valleys. They often look "over-sized" for the small streams that might flow in them today. They can form when meltwater is forced over a col (a ridge) or along the side of a glacier.

3. Depositional Landforms: The "Big Four"

As the glacier retreats and the meltwater loses its energy, it begins to drop its load. This creates several distinctive landforms.

A. Eskers

Imagine a long, winding tunnel flowing underneath the glacier. This tunnel is filled with meltwater carrying sand and gravel. As the glacier melts away, the "floor" of this tunnel is left behind as a long, snake-like ridge of sorted sediment called an esker.

Analogy: Think of an esker like the raised remains of a subway tunnel after the rest of the city has been cleared away.

B. Kames

Kames are mounds or hills of sand and gravel. They form in a few ways, but the most common is when meltwater carries sediment into a "moulin" (a vertical shaft in the ice) or a depression on the glacier's surface. When the ice melts, the pile of sediment collapses onto the ground below, forming a mound.

C. Kame Terraces

These look like flat "sidewalks" or steps running along the side of a glacial valley. They form when meltwater flows in the gap between the glacier and the valley wall. Sediment is deposited there, and when the ice melts, a flat-topped terrace is left behind on the valley side.

D. Outwash Plains (Sandur)

An outwash plain is a large, flat area of sediment found in front of the glacier (the proglacial area). As meltwater exits the snout of the glacier, it spreads out, loses velocity, and drops its load. The largest material (boulders) is dropped nearest the glacier, while fine sand and clay are carried the furthest.

Did you know? Outwash plains often have "braided streams"—small, shallow channels that constantly split and rejoin as they navigate through the piles of sediment they’ve dropped.

Key Takeaway: Eskers are winding ridges; Kames are mounds; Kame Terraces are side-valley steps; and Outwash Plains are broad, flat areas of sediment.

4. Proglacial Lakes and Varves

Sometimes, the glacier or its moraine acts like a dam, trapping meltwater and forming a proglacial lake. Over time, sediment settles at the bottom of these lakes in distinct annual layers called varves.

In the summer, when there is lots of meltwater, a thick layer of light-colored sand is deposited. In the winter, when the lake freezes over and the water is still, very fine, dark-colored clay settles on top. By counting these pairs of light and dark layers, scientists can tell exactly how many years the lake existed!

Math Connection: If you are asked to calculate the age of a lake deposit, remember: \(1 \text{ year} = 1 \text{ light layer} + 1 \text{ dark layer}\).

Glacial meltwater is a crucial part of the glacier landform system. It reminds us that glaciated landscapes are dynamic—they are constantly changing due to the interaction of ice, water, and climate.

  • Causality: Warming temperatures (climate change) lead to increased ablation, which increases meltwater production, leading to the formation of these landforms.
  • Futures and Uncertainties (F): As modern glaciers melt rapidly due to global warming, we may see more frequent "jökulhlaups" (sudden, catastrophic glacial floods) that reshape landscapes in hours rather than centuries.

Quick Review Box:
- Fluvioglacial: Landforms made by meltwater.
- Sorted/Stratified: The key characteristics of meltwater deposits.
- Hydrostatic Pressure: The "secret sauce" that gives subglacial water its erosive power.
- Eskers & Kames: The most common depositional landforms you need to identify.

Don't worry if identifying these in the field seems hard—just remember to look for evidence of water (rounding and layering) vs. evidence of ice (random, angular piles).