By Michael Dallin
The most striking scenery in Rocky Mountain National Park was created by glaciers. Broad valleys, deep cirques and spectacular cliffs are all evidence of glacial erosion. However, the carving of Rocky did not happen overnight -- glacial erosion is an ongoing process, lasting hundreds of thousands of years.
ORIGINS OF GLACIERS
If the climate cools, the summers will no longer be warm enough to melt winter snows. If this happens, the snow accumulates, compacts under the weight, and forms an icy block called a glacier. Glaciers are commonly 1000ft or more thick in places.
Glaciers like to flow downhill. As a glacier moves, it picks up loose boulders and carries them. These boulders scour the underlying rock, grinding it away much like sandpaper on wood. After thousands of years, large gorges are carved out as rock is removed by a glacier. The rocks and boulders a glacier carries is called till.
GLACIERS IN ROCKY
Glaciers play an important role in Rocky's geological history. Snow blown over the Continental Divide collected in east and north-facing valleys and formed glaciers. Glacial valleys are distinctive for their U-shape. Examples include:
Glaciers also carve out steep bowls called cirques. Cirques lie high at the head of a glacial valley, and look as if someone removed the rock with an ice cream scoop. Examples include:
Cirques may appear side by side, separated by thin ridges and a high point, called a horn. The Matterhorn in Europe is a classic example of a horn. Some Rocky Mountain National Park horns include:
WHEN GLACIERS MELT
When a glacier reaches a low enough altitude, the head of the glacier melts. As it melts, it drops the boulders it carries (its till) in large piles and ridges. These ridges are called terminal (end) moraines.
As a glacier moves, some boulders are dropped along the sides of the glacier. These form lateral moraines.
If the climate warms, a glacier may completely melt away, thus dropping its boulders into piles along the bottom of the valley. These piles are called ground moraines.
GLACIATIONS IN ROCKY
Glaciers may have formed in Rocky as long as 300,000 to 750,000 years ago. Little evidence of them remain, since younger glaciers removed all of the older moraines and other glacial features. More recently, the Bull Lake Glaciation occured 130,000 to 150,000 years ago, and the Pinedale Glaciation which began 35,000 years ago and ended roughly 12,000 years ago. These glaciations left moraines across the park, and carved the popular cirques and U-shaped valleys visitors see today. The largest glacier was 20 miles long, and was located in the Never Summer area -- now the headwaters of the Colorado River. In fact, ice covered most of the park, except for the highest peaks, which poked out above the ice.
Since the Pinedale Glaciation, park glaciers have shrunk dramatically. All of them lie inside of their own cirques. Many are no longer active, and are considered permanant snowfields. Some active glaciers include Tyndal Glacier, Andrews Glacier and Rowe Glacier.
So, what makes a glacier active? A glacier is active if it moves downhill. However, glaciers in Rocky move too slow to observe movement. Observers can detect movement by finding crevasses along the glacier.