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Paleo Slide Set: The Ice Ages
Striations on rock, Baffin Island, Nunavut, Canada.
There were still other observations besides the erratic formations and U-shaped valleys. In the same regions where erratics occurred, naturalists noted exposed bedrock that sometimes bore parallel scratch marks (such as these grooves oriented vertically on the right side of this photograph), termed striations. The ice-age theory states that striations form when boulders trapped at the base of ice sheets scratch across bedrock, leaving deep scratch marks.

When geologists began to take note of striations, erratics, and U-shaped valleys in the eighteenth and early nineteenth centuries, they were puzzled by these phenomena. In an age when science and religion were closely connected, most attributed these features to the great flood described in the Bible. But by the 1840s, a growing number of geologists were abandoning The Diluvian Theory in favor of a hypothesis put forth most stridently by Louis Agassiz.

By the middle of the 19th century, most geologists were convinced by Agassiz's theory that past periods of more intense glaciation explained the presence of features like striations and erratics in regions far removed from present-day glaciers. With the existence of ice ages no longer in doubt, scientists began to wonder what caused such striking climate changes.. Some theorized that the amount of energy produced by the sun had varied in the past causing climatic oscillations. Others speculated that ash hurled into the atmosphere by volcanic eruptions could have blocked the sun's radiation and thrust the globe into a protracted cold spell. British geologist Charles Lyell proposed that ice ages occurred because of an increase in the elevation of the earth's crust. On the other hand, the French astronomer J. A. Adhémar suggested that variations in the earth's orbit around the sun might account for climatic fluctuations.

Many of these theories, however, were soon disproved. If increased volcanic activity had directly caused the ice ages, then sediments deposited during ice ages should have contained sizable ash layers. But the geologic record failed to support the volcanic theory, just as it failed to substantiate the notion that variations in solar output dictated climatic oscillations or Lyell's scheme of increased crust elevation. Adhémar had also made a crucial mistake, arguing that the Northern Hemisphere was heating up while the Southern Hemisphere was cooling down, and vice versa. It was easily demonstrated that each hemisphere received the same exact amount of heat as the other over the course of the year.

Photo Credits:
John T. Andrews
INSTAAR and Department of Geological Sciences, University of Colorado, Boulder.

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Last Modified: 12 October 2001

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