Glacial Depositional Landforms: Moraines, Drumlins, and Eskers

Glacial depositional landforms are features built from sediment dropped by ice and meltwater, including moraines (ridges of till), drumlins (streamlined, egg-shaped hills), eskers (long winding ridges of sand and gravel), erratics, outwash plains and kettle lakes. In brief: the material dropped directly by ice is called till, and it is unsorted and angular, with clay, sand and boulders mixed together. Material carried and sorted by meltwater is outwash (glaciofluvial deposits), and it is layered and rounded. Terminal moraines mark the furthest advance of a glacier, drumlins show the direction of ice flow, and eskers record the path of streams under the ice.

These landforms cover large areas of northern Europe and North America and give a record of the last ice age. This guide explains the types of deposit, describes each major landform and how it forms, compares them in a table, works through calculations, outlines the history of their study and corrects common mistakes.

What sediments do glaciers deposit?

Glaciers carry rock debris on the surface (supraglacial), within the ice (englacial) and at the base (subglacial). When the ice melts or loses energy, the load is deposited. Till (boulder clay) is deposited directly from the ice. It is unsorted, with fragments of all sizes from clay to boulders, and the stones are usually angular. Glaciofluvial (outwash) deposits are laid down by meltwater streams, so they are sorted by size and layered, and the stones are more rounded. This difference helps geologists to tell the origin of deposits.

What are moraines?

Moraines are ridges or sheets of till. The main types are:

• Lateral moraines: ridges along the sides of a valley glacier, made from debris that has fallen from the valley walls.
• Medial moraines: a dark stripe in the middle of a glacier where two lateral moraines join when two glaciers merge.
• Terminal (end) moraines: a ridge of till at the farthest point reached by the ice, where it stayed in one place long enough for material to pile up. Long Island in New York is largely a terminal moraine of the last ice age.
• Recessional moraines: smaller ridges left behind when the retreating glacier paused.
• Ground moraine: a layer of till spread over the land beneath the ice, forming a rolling surface.

What are drumlins and erratics?

Drumlins are smooth, elongated hills of till, shaped like the back of a spoon turned upside down, often 250 to 1,000 m long, up to about 500 m wide and 5 to 50 m high. The steep (stoss) end faces the direction the ice came from, and the gentler tail points in the direction of flow. They occur in groups called drumlin fields or “baskets of eggs” topography, as in Ireland (Clew Bay), northern England and New York State. Their origin is still debated: they may have formed when the ice moved over and molded the till, or by erosion or by floods beneath the ice.

Erratics are large boulders carried by ice and left in an area with a different rock type. Their origin can be traced to the source, which shows the ice flow. Shap granite boulders, for example, were carried out of the Lake District over long distances.

What landforms do meltwater deposits form?

• Eskers: long, narrow, winding ridges of sand and gravel, from a few meters to tens of meters high and sometimes many kilometers long. They form from sediment laid down by streams in tunnels beneath or within the ice and appear as the ice melts. They are common in Canada, Sweden and Finland.
• Kames: mounds of sand and gravel, deposited by meltwater at the edge of the ice or in holes on its surface.
• Outwash plains (sandurs): broad, flat plains of sand and gravel, deposited by braided meltwater streams beyond the terminal moraine, as in southern Iceland.
• Kettle holes and kettle lakes: depressions left when blocks of ice that were buried in outwash melted, often filled with water.
• Varves: layers of fine sediment in glacial lakes, with a light summer layer and a dark winter layer, that can be counted like tree rings.

Depositional landforms compared

Worked example: dimensions and distances

The numbers are hypothetical, to show how field measurements are used.

1. Drumlin shape: a drumlin is 600 m long, 200 m wide and 25 m high. The length-to-width ratio (elongation) is 600 ÷ 200 = 3. Drumlin fields often have ratios of 2 to 4 or more.
2. Drumlin volume: approximating a drumlin as half of an ellipsoid, the volume is (2 ÷ 3) × π × (600 ÷ 2) × (200 ÷ 2) × 25 = (2 ÷ 3) × π × 300 × 100 × 25 = about 1,570,000 m³.
3. Number in a field: a field of 100 km² has 1,000 drumlins, a density of 1,000 ÷ 100 = 10 per km².
4. Esker length and age: an esker 120 km long, formed behind a retreating ice margin that withdrew at 100 m per year, would be built over 120,000 ÷ 100 = 1,200 years if it formed progressively along the full length.
5. Retreat rate from moraines: two recessional moraines are 3 km apart and are dated to 400 years apart. The average retreat is 3,000 ÷ 400 = 7.5 m per year.
6. Erratic transport: an erratic of granite lies 80 km from its source outcrop. If the ice moved at 40 m per year, the journey took 80,000 ÷ 40 = 2,000 years.

How did we learn about these deposits?

The unsorted clay and boulders covering much of northern Europe and North America puzzled early geologists. In the 19th century, the term “drift” was used because it was thought that the deposits had been carried by floods or floating icebergs. Agassiz’s glacial theory and later fieldwork showed that land ice was responsible. Detailed maps of moraines, eskers and drumlins allowed reconstruction of the extent and flow of the ice sheets, and radiocarbon and other dating methods gave ages. The last ice sheets reached their maximum extent about 20,000 years ago and retreated from most of the lowlands by about 10,000 years ago. The landforms also help locate sand and gravel for construction, and they determine drainage and soils.

Which mistakes are common?

Mistake 1: Mixing up till and outwash

Till is unsorted and was deposited by ice. Outwash is sorted and layered and was deposited by water.

Mistake 2: Thinking that drumlins point the wrong way

The steeper, blunt end faces upstream, toward the direction the ice came from, and the gentler tail points downstream.

Mistake 3: Confusing eskers with moraines

Eskers are winding ridges of sorted sand and gravel from meltwater streams. Moraines are ridges of unsorted till.

Mistake 4: Believing that an erratic is a local boulder

An erratic differs in rock type from the bedrock where it lies, and it was transported by ice from elsewhere.