Where ONCE were Glaciers
Louis Agassiz, was primarily a biologist (who, by the way, was not very fond of the radical and crazy theory of his day, called evolution), who lived in the mid-1800’s (1807-1873); born in Switzerland, emigrated to the U.S. where he became a professor at Harvard.
His greatest contribution to science though, was not biological but geological.
He noted that many valleys in the European Alps seemed unusually large and deep for the streams that were found in their centers. He also recognized huge piles of unconsolidated material that seemed unlikely to have been moved by those small streams.
He decided that something else must have cut these valleys, and realized that it must have been the expansion of glaciers whose remnants are now at the head of the valleys.
In other words, Agassiz recognized that there were periods in earth’s past where cold temperatures greatly enhanced the growth of glaciers.
Cold periods not only enhanced the relatively small glaciers in the Swiss Alps (and other mountain highlands), but also enormous ice masses that we have come to know as “Continental Ice Sheet Glaciers.”
On the left, a Continental Ice Sheet in Greenland, and on the right a valley glacier in the Alps, much like the upper reaches of the valleys that Agassiz visited in the 19th century.
On the left, a map of terminal (end) moraines that were noted to have rather enormous size and extent. These terminal moraines in the Great Lakes region (and beyond) of the United States were the “end points” of an enormous ice mass that worked down from the north.
And, below right, the geological interpretation of these moraines– an expansion of a continental ice sheet, called the Laurentide, which reached a maximum extent around 18-20ky (thousand years ago).
Cool Video on Melting of the Laurentide Ice Sheet
(as you’ll see– What’s left of it?=> Greenland Glaciers)
So, how do glaciers grow and shrink? Ice in a glacier is almost always moving downhill. The only real exception to this would be the massive continental ice sheets that are able to establish a pressure-head capable of driving a glacier over topographic rises.
Glaciologists have come to recognize that a glacier moves, grows, and shrinks in accordance with a kind of “budget.” There is the money-making part of the glacier, i.e. the zone of accumulation where snowfall exceeds melt. There is the spending or losing end of the glacier, called the zone of ablation where melt exceeds rejuvenation. Between these is a line (or perhaps better thought of as a zone) called the equilibrium line.
In the case of a mountain range that I really love, glaciers (and ice and snowfields in general) are “losing the battle” and getting smaller each year.
Below is an image of the Grand Teton with its tiny glacier (the Teton Glacier) located on the north (right side) of the big peak. You can see that there is a wall of grey colored rocky sediment (recessional moraine) in front of the Teton Glacier, and a U-shaped valley coming down towards the foreground.
Rising above the Teton Glacier is one of the great ice climbs of North America, the Black Ice Couloir (see image on the right). Unfortunately, the ice in this ultra-steep gully is rather infrequently forming in today’s warming environment.