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. Adhemar 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. Adhemar 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.