Glaciation evidence helps students identify where glaciers once existed and how ice reshaped Earth’s surface. This cheat sheet covers erosional landforms, depositional features, and evidence used to connect glaciers to past climate change. Students need these clues to interpret maps, landscapes, sediment layers, and geologic history.
It is especially useful for comparing local landforms with global ice-age patterns.
The core ideas include recognizing striations, U-shaped valleys, cirques, moraines, drumlins, eskers, and glacial erratics. Erosional evidence forms when moving ice scrapes, plucks, and carves rock. Depositional evidence forms when melting ice drops unsorted sediment called till or sorted sediment from meltwater.
Ice-age timing is linked to changes in Earth’s orbit, tilt, and wobble, known as Milankovitch cycles.
Key Facts
- Glacial striations are parallel scratches in bedrock that show the direction of ice movement.
- A U-shaped valley forms when a glacier widens and deepens a stream-cut V-shaped valley.
- A cirque is a bowl-shaped hollow carved near the head of a mountain glacier.
- Till is unsorted glacial sediment containing mixed particle sizes from clay to boulders.
- A moraine is a ridge or pile of till deposited at the edge, end, or middle of a glacier.
- A drumlin is a streamlined hill of till that usually points in the direction of ice flow.
- An esker is a winding ridge of sorted sand and gravel deposited by meltwater flowing inside or beneath a glacier.
- Milankovitch cycles include eccentricity, obliquity, and precession, which affect how sunlight is distributed on Earth over long time periods.
Vocabulary
- Glacial erosion
- The wearing away of rock and soil by moving ice through abrasion and plucking.
- Abrasion
- The scraping and grinding of bedrock by rock fragments frozen into the base of a glacier.
- Plucking
- The process in which a glacier freezes onto broken rock and pulls pieces away as the ice moves.
- Till
- Unsorted sediment deposited directly by glacial ice.
- Erratic
- A rock transported by a glacier and deposited in an area where it does not match the local bedrock.
- Milankovitch cycles
- Long-term changes in Earth’s orbit, axial tilt, and wobble that influence climate and ice-age timing.
Common Mistakes to Avoid
- Confusing till with meltwater deposits is wrong because till is unsorted, while meltwater deposits are usually sorted by particle size.
- Assuming all scratches on rock are glacial striations is wrong because striations must be parallel, polished into bedrock, and consistent with ice-flow evidence.
- Mixing up U-shaped and V-shaped valleys is wrong because glaciers carve broad U-shaped valleys, while rivers usually carve narrow V-shaped valleys.
- Thinking a single landform proves an ice age is wrong because geologists use multiple lines of evidence, such as striations, moraines, erratics, and sediment layers.
- Saying Milankovitch cycles directly cause instant climate change is wrong because they change sunlight patterns gradually and interact with oceans, atmosphere, ice, and carbon cycles.
Practice Questions
- 1 A bedrock surface has parallel scratches trending northwest to southeast. What type of glacial evidence is this, and what can it tell scientists?
- 2 A deposit contains clay, sand, pebbles, and boulders all mixed together with no layering. Is this more likely till or a meltwater deposit? Explain your choice.
- 3 A glacier leaves a ridge of sediment at its farthest downhill position. What type of moraine is this, and what does it mark?
- 4 Why do scientists use both erosional evidence and depositional evidence when reconstructing the size and movement of ancient glaciers?