Past and Present Ice Cover

Understanding the history of glaciation — and the evidence used to reconstruct it — is fundamental to A-Level Geography. The Earth has experienced repeated glacial episodes over geological time, with the most recent major period (the Pleistocene) shaping much of the landscape we see today in the UK and across the globe. This lesson examines the causes of glaciation, the evidence for past ice cover, and the current state of the world's ice.

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The Pleistocene Epoch

The Pleistocene epoch lasted from approximately 2.6 million years ago to 11,700 years ago. It was characterised by repeated cycles of glacial and interglacial periods:

• Glacial periods (ice ages) — lasting approximately 80,000–100,000 years; global temperatures 5–8°C lower than today; massive ice sheets covered large areas of the Northern Hemisphere
• Interglacial periods — lasting approximately 10,000–30,000 years; temperatures similar to or slightly warmer than today; ice sheets retreated to polar regions
• We are currently in an interglacial period — the Holocene — which began approximately 11,700 years ago

UK Glacial Chronology

Glaciation	Approximate Date	Extent
Anglian	~450,000 years ago	Most extensive British glaciation; ice reached the Thames valley in south-east England
Wolstonian	~300,000–130,000 years ago	Less extensive; ice covered much of northern and central England
Devensian	~115,000–11,700 years ago	Last major glaciation; ice covered Scotland, northern England, Wales, and most of Ireland
Devensian Maximum	~26,000 years ago	Ice reached approximately the line of the Severn-Wash (the Devensian limit)
Loch Lomond Stadial	~12,900–11,700 years ago	Brief cold snap; small corrie glaciers reformed in Scottish Highlands, Lake District, and Snowdonia

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Causes of Glaciation: Milankovitch Cycles

The Serbian mathematician and geophysicist Milutin Milankovitch (1941) proposed that glacial-interglacial cycles are driven by predictable variations in Earth's orbit around the Sun. These Milankovitch cycles affect the amount and distribution of solar radiation (insolation) reaching the Earth.

Cycle	Description	Period	Effect
Eccentricity	Change in shape of Earth's orbit from nearly circular to more elliptical	~96,000 and ~413,000 years	Affects total annual insolation received; when orbit is more elliptical, seasonal differences in insolation are greater
Obliquity (axial tilt)	Change in the tilt of Earth's axis (currently 23.4°; ranges from 22.1° to 24.5°)	~41,000 years	Greater tilt = more extreme seasons; less tilt = milder seasons and cooler summers (favouring snow accumulation)
Precession	Wobble of Earth's axis (like a spinning top) changes the timing of perihelion relative to the seasons	~26,000 years	Affects the seasonal distribution of insolation; determines whether Northern Hemisphere summer coincides with perihelion or aphelion

graph TD
    A["Milankovitch Cycles"] --> B["Eccentricity<br/>~96,000 / 413,000 yr"]
    A --> C["Obliquity<br/>~41,000 yr"]
    A --> D["Precession<br/>~26,000 yr"]
    B --> E["Combined effect on<br/>seasonal insolation<br/>at high latitudes"]
    C --> E
    D --> E
    E --> F["Cool summers at<br/>high latitudes"]
    F --> G["Snow survives summer<br/>→ Ice sheets grow<br/>→ Glacial period"]

Key Point: The critical factor is cool summers in the Northern Hemisphere — not cold winters. Cool summers prevent winter snow from melting completely, allowing it to accumulate year on year. Milankovitch's theory explains the timing of glacial cycles but not their full amplitude — feedback mechanisms (especially ice-albedo feedback and CO₂ changes) amplify the orbital signal.

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Evidence for Past Glaciation

How do we know that ice sheets once covered areas that are now ice-free? The evidence comes from multiple sources:

Geomorphological Evidence

Evidence	What It Tells Us
U-shaped valleys	Direction and extent of valley glaciers
Corries and arêtes	Locations of former glaciers; predominant aspect indicates climate conditions
Drumlins	Direction of ice flow (stoss end faces upstream)
Moraines	Extent of glacier advance (terminal moraines mark maximum limits)
Erratics	Direction and distance of ice transport; matching lithology to source
Striations	Direction of ice movement at specific locations
Roches moutonnées	Direction of ice flow (smooth stoss, rough lee)

Sedimentological Evidence

• Till deposits — unsorted, unstratified material indicates direct glacial deposition
• Varves — annual layers in proglacial lake sediments provide chronological records (De Geer, 1912)
• Loess — fine wind-blown silt deposited around glacier margins; extensive in central Europe and China

Biological Evidence