While taking a wilderness stroll or relaxing on a scenic drive, it’s easy to wonder, “What created this beautiful mountain landscape?” The Rocky Mountains as we know them are not all that ancient, only about 70 million years old. Much of the mountain building in this area was the result of violent earthquakes that caused blocks of earth to rise vertically to form mountains. Through uplift, weathering and erosion, very old rocks have been exposed from the depths of the earth. Some of the rocks from which our mountains are comprised are as old as 1.7 billion years.
So when looking out at a landscape, go ahead and make an interpretation. Ask yourself, “What kind of rock am I looking at?” An easy rock type to recognize is sandstone. Sandstones are often tan or reddish in color, and often form cliff bands, such as the cliffs outside of Minturn. Sandstone is a sedimentary rock composed of grains of sand, and since Colorado was covered by ocean for hundreds of millions of years, there was plenty of time for sand to be deposited on beaches. Other sources of sandstone include sand dunes from ancient deserts, and sandbars from ancient rivers. Go ahead, and let your mind wander through the realm of geologic possibilities.
Since sediments are often laid down horizontally, look for bands of layering within a cliff or mountainside. Often mixed in between the layers of sandstone are layers of limestone. Limestone can be recognized by its bland gray color. It is formed from an accumulation of shelled sea creatures which once thrived here in the shallow ocean.
Most of us can recognize granite because we’ve seen it polished smooth for countertops, or as facing on fancy city buildings. Granite is speckled pink, white, gray, and black. The large crystals tell us that granite was formed by molten rock that was squeezed up from deep within the earth and then cooled slowly beneath the ground. Granite is common in the Gore Range, and can be seen where it was polished smooth from alpine glaciers.
When you look out at Mount of the Holy Cross and other rugged high peaks in the Sawatch Range, you’ll see the rock appears black. These metamorphic rocks are very old Precambrian gneiss. Metamorphic rocks are formed when a parent rock is heated, squeezed and compressed under intense pressure from major earth forces. Although the rock is never melted, its character changes. In the Sawatch Range, ancient marine deposits were transformed into gneiss, a black rock beautifully striped with wavy red, gray, and white bands.
In the arid parts of western Eagle County, you’ll find gray powdery shale deposited in thin crumbly layers. This rock does not absorb water well and it is not uncommon to have mudslides during periods of melting, or heavy rains. Imagine an ancient river hitting the ocean and the smallest particles drifting to the deepest depths, this is how shale is deposited.
Western Eagle County is also home the beautiful Castle Peak. The unique appearance of this peak is due to its volcanic origin. Take a drive up Colorado Highway 131, and as you go over the pass toward State Bridge, notice the black volcanic rock with the large pockmarks. The holes were formed by escaping gases as the molten rock was hardening.
In most recent geologic history, we emerged from the ice age a mere 10,000 years ago. For tens of thousands of years the earth was colder overall and glacial ice covered our mountains. Our glaciers did not come from the north, but rather they grew from the mountaintops as alpine glaciers. Gravity pulled, and the heavy alpine glaciers gouged their way down high mountain valleys, plucking giant boulders from the landscape along the way, like bulldozers of ice.
About 10,000 years ago Earth’s climate warmed. We’re talking, “The Big Spring Runoff”. The melt water from the vast quantities of ice and snow caused huge floodwaters. Enormous rivers carrying huge rolling boulders and blocks of ice carved down through solid rock to form deep and spectacular canyons, and then later deposited a wealth of soils and gravels in the broad river valleys below.
So to answer the question, “What formed this beautiful landscape?” The next time you look at sandstone cliffs, think of a beach or even sand dunes. When you see an arid landscape of shale, think of a deep ocean. Limestone boulders? Think of seashells. Eyeing a saw-toothed ridge on the skyline? Think glaciers.
The ultimate answer is time has formed this landscape, and this complex landscape continues to evolve each day.