By Michael Dallin and Donald Lyon
There is no question that the topography of Rocky Mountain National Park has been dramatically altered by glacial action. Almost every peak in the Park was touched by glaciers at some point, and most of the lakes are a direct result of glacial activity. But where did these ancient ice rivers come from?
FORMATION OF GLACIERS
Glaciers are formed in cool and wet climates. As the summers cooled, winter snows did not have adequate time to melt. As the yearly snows piled up, they were compressed into ice by the weight of new snows, forming ice fields. The Park still contains many ice fields and other permanent snowfields.
To consider an ice field a glacier, the ice field must move -- that is, it must flow downhill. As the weight of the ice field increases from new snows, the pressure coupled with gravity causes the ice field to move. The ice field is now a glacier. More specifically, the ice field is now a Valley (or Alpine) glacier. Continental Glaciers, which cover hundreds of square kilometers of land, did not occur within the Park.
As a glacier moves downhill, the areas at the top (that is, the source) of the glacier continue to replenish. In the Park, prevailing winds blow snow up and over the Continental Divide, depositing it at the heads of the glaciers. Since the prevailing wind blows west to east, move of the heads of the glaciers were (and still are) in east-facing valleys.
Glacial movement (also called basal sliding) is detected by crevasses in the glacier. Much like water in a river, glaciers speed up over steep sections of rock, and slow down over gentle slopes. However, the ice can not stretch, and the pull of the faster moving sections causes the ice to crack, forming crevasses. As the ice slows down, compression closes the crevasses. Crevasses are also formed as the glacier curves, since the ice on the outside of the curve flows faster than the ice on the inside.
Occasionally, a glacier may pull away from the headwall, forming a crevasse called a bergschrund. The Park glaciers today develop bergschrunds, except during periods of drought. A hike to the summit of Flattop Mountain usually provides the best views of a bergschrund atop Tyndal glacier.
GLACIAL EROSION
Glacial movement wears away the rock underneath the glacier. Water flows under glaciers, giving the ice a lubricated surface upon which to move. This water also seeps into cracks in the rock, and eventually refreezes. As water the water freezes it expands, causing the cracks to widen and the rock to loosen. As the glacier moves it picks up these loose rocks. As these rocks are pulled downhill by the glacier, they grind away at the underlying bedrock, polishing it, much like sandpaper. This grinding leaves striations on the bedrock, many of which are visible in the Park today.
GLACIAL LANDMARKS
However, the most impressive aspects of glaciers are the landmarks they leave behind. The most obvious are the "U-shaped" valleys, and Rocky Mountain National Park is full of them. As a glacier flows downhill, the weight and pressures of the ice tend to "flatten" or broaden the valley -- hence, it creates the characteristic U-shape. Rivers, on the other hand, tend to cut straight downward, creating a V-shaped valley, which can be seen at lower elevations in the Park. Many of the U-shaped valleys started as V-shaped before glacial erosion broadened the valley floors. Forest Canyon, the Fall River Valley, the Cache la Poudre River Valley and the Kawuneeche (Colorado River) Valley are perhaps the best examples of U-shaped valleys.
Glaciers may have ice tributaries that flow into them. These tributaries are not as deep as the trunk glacier, and hence they do not dig as deep. After the glacier retreats, the valleys formed by the tributaries run at an angle to the main glacial valley. Since the tributaries can not dig as deep as the massive trunk glacier, the entrance to the tributary valley (called a "Hanging Valley") are above the main glacial valley floor. Rocky Mountain National Park has many hanging valleys, the most obvious one named Hanging Valley -- it is the valley just west of Rainbow Curve on Trail Ridge Road, and the road takes you around the rim of it.
Of particular intrest to hikers are Rock Basin Lakes. The appear when a glacier flows down a hill made up of cracked and broken rock. Water seeps into the cracks and then freezes, causing the cracks to expand. Hence, the rocks are broken away along the cracks, forming steps of rock. After the glacier retreats, lakes may appear on the steps left behind. These lakes are rock basin lakes. Emerald and Dream Lakes are perhaps the most visited rock basin lakes in the Park. Other visible lakes are Chasm Lake and Peacock Pool below Longs Peak, and the Gorge Lakes visible across Forest Canyon from Trail Ridge Road.
The head of a glacial valley is a spectacular place to view the most impressive feature of glaciers: cirques. A cirque is the area where a glacier first forms. It is a bowl shape, reminiscient of an ice cream scoop removing part of a mountain. The rock walls above a glacier are extremely steep, and are formed by the freeze/thaw cycle known as frost wedging. Often, bergschrunds appear at the head of a cirque. The most famous cirque in the Park is the Longs Peak/Mount Meeker cirque. The best way to view it is a hike to Chasm Lake. Other easily viewable cirques are those along the Continental Divide in the east side of the Park -- including Chaos Canyon, Tyndal Gorge and the cirques above Loch Vale.
As glaciers grow, the ridges above them shrink. Eventually, a thin ridge will separate two glaciers. This ridge is called an Are^te (pronounced Uh-Rate) The Arrowhead/McHenry's Peak ridge is an are^te. As the glaciers cut closer to each other, they tend to carve out the induvidual sides of a peak, forming a Horn. Horns are very steep and pointed peaks. The most famous horn in the world is the Matterhorn. A good Park example of a horn is Pagoda Mountain, southwest of Longs Peak.
When a glacier melts, it will occasionally leave small blocks of ice half-buried in the ground. Eventually these blocks will melt, and the hole left behind will fill with water, forming small lakes called "kettle lakes". Sheep Lakes in Moraine Park are a good example of kettle lakes, and are very visible from Rainbow Curve on Trail Ridge Road. Near the Sheep Lakes are "kames", which are rounded hills of glacial debris deposited by meltwater streams.
MORAINES
Just as a glacier plucks rocks from the bedrock and carries them downhill, they must eventually drop them. The rocks dropped by a glacier are usually unsorted, and are called "till". As more and more rocks are dropped, long ridges form. These deposited ridges are called moraines. Moraines may be one of the most subtle glacial features, unless you know what to look for. Moraines are abundant in the Park -- in fact, the Bear Lake parking lot is build on one, and the Bear Lake road climbs one. They are striking in Moraine Park, and wonderful to view from Many Parks Curve on Trail Ridge Road.
However, moraines are more than just large piles of rocks. A moraine can be created in a variety of ways, and each way creates a uniquely different moraine.
Lateral Moraines: As a glacier flows downhill, rocks tend to accumulate along the sides. Most of this accumulation is a result of the rocks from the sides of the valley falling onto the edge of the glacier. Eventually, these rocks form what is called a "lateral moraine". The South Lateral Moraine, located on the southern end of Moraine Park, is a classic lateral moraine.
End Moraines: These are created at the terminus of a glacier. As a glacier moves, it pushes debris, and when the debris reaches the end of the glacier, it piles up into a ridgeline of till along the front edge of the glacier. These end moraines help determine the shape of the glacier, and are usually crescent-shaped. There are two types of end moraines: terminal moraines and recessional moraines.
Terminal Moraines: These are end moraines that appear at the furthest advance of a glacier. A terminal moraine is just east of the Bear Lake Road at Moraine Park.
Recessional Moraines: As a glacier grows and retreats, it creates new end moraines, called "recessional moraines". These moraines do not reach as far as the terminal moraine. The road through Horseshoe Park crosses small recessional moraines between Sheep Lakes and the parking lot on the east end of Horseshoe Park.
Ground Moraines: When a glacier melts, any rocks on top of or inside of it drop, littering the ground with large patches of till. The floor of Moraine Park is a prime example of ground moraines.
Morainal lakes: Many times moraines block the flow of meltwater from glaciers, forming lakes. Horseshoe Park and Moraine Park were once covered with these types of ancient lakes.
GLACIAL ERAS
Throughout the last million years or so, the earth's climate has flucuated, causing the glaciers to periodically advance and retreat. Periods of advance are called glatiations. Each glaciation leaves evidence of its passing -- the most obvious being moraines. These and other features are used by geologists to paint a picture of the Park's natural history.
PRE-BULL LAKE
The oldest glaciations are called the "Pre-Bull Lake" glaciations. Very little evidence of these ancient glaciers remain, as other (more recent) glaciers have destroyed many of the features left by them. The Pre-Bull Lake glaciation may have begun as much as 5 million years ago (though 500,000 years ago is more likely), and ended around 245,000 years ago. Evidence of the Pre-Bull Lake glaciers can be found east of the Park. These glaciers were the largest in the area, and they formed the basis for what would later become the major valleys in the Park. This is generally considered the wettest and coldest of the glacial periods.
BULL LAKE
The Bull Lake glaciation began roughly 160,000 years ago, and lasted for 40,000 years. The Bull Lake glaciation is broken into three parts: Early, Middle and Late Bull Lake. Between these periods were times of relative warmth, causing the glaciers to retreat.
PINEDALE
The Pinedale glaciation occured 35,000 years ago, and lasted approximately 20,000 years. The Pinedale glaciations are broken into three parts, just as the Bull Lake glaciations.
MODERN GLACIERS
Since the Pinedale glaciers retreated, there have been several small glaciations. The latest is termed the Arapaho Peak glaciation, and its farthest advance was in the 1850's. However, none of the glaciations can compare in magnitude to their ancient ancestors.
The glaciers in the Park today are remnants of the Arapaho Peak glaciation. While these glaciers are small, they still move. The most visible are Tyndal, Andrews and Taylor glaciers. Other more remote glaciers are Rowe and Sprague glaciers.
Rocky Mountain National Park