Icebergs and glaciation

Ice – particularly in the form of glaciers – has had a significant impact on landforms and life forms round the globe. And studies of ice and glaciers have made a significant contribution to our understanding of biogeography.

Glacier boulder

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This landscape in Nunavut, Canada, shows the impact of glacier erosion. There is very little on the bare granite rock surface, though there is a large boulder that would have been carried by a glacier and then deposited.

Rights: Lynda Dredge of the Geological Survey of Canada

Early views on glaciation

When geographers first began to document where ice was found on the Earth, they thought that icecaps had always been found only round the North and South poles, plus a few glaciers and snowfields on particularly high mountains. This agreed with the prevailing view that the Earth had been created pretty much in its present form only a few thousand years earlier.

Scientists had noticed that, in some areas, there were large boulders that were quite different from the rest of the local geological features. These “erratic” boulders had obviously been carried quite some distance from their source. They were generally believed to have been taken there by the rushing waters of the Biblical Flood, or else frozen in icebergs carried by that same flood.

Glacier striations

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By running a finger along freshly exposed striations one can determine the direction of the ice flow by the feel of the rock. The moving ice leaves the rock grains aligned with the direction of movement, so when the striation feels smooth, your finger is moving in the direction of ice flow and when it feels rough, you are moving against the ice flow.

Rights: Geography Site

In the early 18th century, a Swiss mountaineer, Jean-Pierre Perraudin, noted that even very hard rocks appeared to have been scarred and scratched by something massive, and equally hard, pressing down on them. He suggested that these marks had been caused by boulders carried along on the underside of ancient glaciers – in other words, areas of the modern Earth that were ice-free had been glaciated in some earlier time. At first, this idea was not generally accepted, even though the scarred and scraped rocks were widespread. Opponents even suggested that the marks were grooves worn by passing carts and coaches!

Perraudin’s idea was picked up by two engineers who were interested in geology, Ignace Venetz and Jean le Charpentier. Venetz recognised that the presence of glacial moraines miles from the face of any existing glacier implied that glaciers had extended much further in the past. Charpentier gave talks about the glacial hypothesis to scientific audiences and, in 1834, attracted the attention of geologist Louis Agassiz, who thought the whole idea ridiculous and set out to disprove it.

But Agassiz found so much evidence in support of the idea of widespread past glaciations that he was converted and, in fact, took things to extremes, arguing that the entire Earth had once been ice-bound, and that this would have killed off every living thing then alive. This reflected his acceptance of the idea of catastrophism, whereby life on Earth was destroyed by global catastrophes – and subsequently replaced through a Creation event.

Another scientist to accept that there had been at least one Ice Age in the Earth’s past was Charles Darwin’s friend and mentor, Charles Lyell. To him, the idea that geological features could be shaped by processes like glaciation fitted well with the concept of uniformitarianism that the Earth is and always has been subjected to ongoing and unchanging processes of gradual change e.g. erosion and sedimentation. Needless to say, Lyell was not a fan of catastrophism.

Once the concept of Ice Ages had been accepted, scientists turned to considering what had caused these much colder periods in the Earth’s past.

In the early 20th century, it was recognised that the evidence for glaciation around the globe could also provide evidence for the movement of the continents. This was a big change, as scientists had previously believed that the position of the continents was fixed – it took some considerable time until the new idea was accepted. In fact, this really only happened once geologists proposed a mechanism for moving the continents.