We live in a beautiful part of the country, and its in large part thanks to ice.
The area of the present day Animas Valley once contained about 2,000 feet of glacial ice that nearly reached the top of the cliffs. This was during the Pinedale glaciation period, between 12,000 and 23,000 years ago. The glacier extended for 40 miles. Average air temperatures were much colder than current day. Snow collected faster than it melted even in the summer. The snow compressed into ice. Vast areas were frozen underneath for hundreds, maybe thousands of years.
Two thousand feet of ice has the power to crush rock. Add a slight slope, and gravity makes the entire icy chunk move along like a gigantic bulldozer. Glaciers generally move very slowly sometimes immeasurably but have the force to literally move mountains.
Many smaller glaciers carved away rock in the San Juan Mountains during glacial periods. No moving glaciers remain in the Colorado Rockies, but proof of their presence is strikingly visible in the various glacial landforms.
Some of the many landforms created by these glaciers were glacial erratics. Are you wondering what a glacial erratic is? Of course, a photo is worth a thousand words, and one place to find photos of glacial erratics is in the March 2012 issue of National Geographic.
Glacial erratics are the most noticeable part of the ground moraine. Ground moraine is the debris left behind underneath a glacier. Most ground moraine is barely noticeable because after thousands of years it is mostly covered by vegetation.
Exceptions here are glacial erratics. Erratics are giant boulders deposited by glaciers. These boulders are smooth, unlike jagged rocks that break and fall off mountains.
They can be found in places where there are no other boulders and thousands of miles from other rocks of similar composition. Erratics are just one of the many glacial landforms noticeable in the San Juan Mountains.
MK Thompson is conservation education assistant for San Juan Mountains Association. SJMA is a nonprofit dedicated to public land stewardship and education.