Mills Lake Erratic EarthCache
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Rocky Mountain National Park is located west of Estes Park and north and east of Grand Lake. This is a fee area of the National Park Service, and costs $30 per vehicle. This fee is covered in the Rocky Mountain National Park Annual Pass, the Rocky Mountain National Park/Arapaho National Recreation Area Annual Pass, and the America the Beautiful Pass. Please see the following website (visit link) for the entire fee schedule. The park is open 24 hours a day, 365 days a year. Road and Trail Conditions and Closures can be found at: (visit link) Recorded information for the condition of Trail Ridge Road can be found by calling (970) 586-1222. Please remember that all geologic features within the borders of Rocky Mountain National Park are protected by law, as are all natural and historic features. Please do not disturb, damage, or remove any rocks, plants, or animals.
Parking is located at the Glacier Gorge Trailhead, or you can park at one of the several shuttle bus parking lots and take the shuttle to the trailhead.
The given coordinates will lead you to a glacial erratic on the shore of Mills Lake. A glacial erratic is a boulder that has been transported from its place of origin by the movement of ice. When the glacier melted or retreated, the erratic was left behind.
The erratic at this site is nearly spherical, and formed of pink granite from the Longs Peak-St. Vrain Batholith. This puts the age of the granite on the order of 1.4 billion years old.
Using a ruler or tape measure, find the diameter of this erratic. Assuming that this material has a density of 2.75 grams per cubic centimeter (gm/cm3), how much does this erratic weigh?
1.) To receive credit for this EarthCache, send me an email with measurements that you took for this erratic, along with its weight.
2.) Is this erratic made of similar material or different material than the rock of the lakeshore upon which it sits? Explain similarities or differences in your email to me.
Please consider posting photos of yourself, or the local geology, when you log this EarthCache. Photos can be an additional rewarding part of your journey, but posting them is not a requirement for logging this EarthCache, and is strictly optional.
The above information was compiled from the following sources:
2004. Rocky Mountain National Park. In Harris, A.G. et al., editors. Geology of National Parks, Sixth Ed. P. 337-356. Dubuque, Iowa: Kendall/Hunt Publishing Company.
Cole, J.C., and Braddock, W.A. 2009. Geologic map of the Estes Park 30’ x 60’ quadrangle, north-central Colorado: U.S. Geological Survey Scientific Investigations Map 3039, 1 sheet, scale 1:100,000, 1 pamphlet, 56 p.
KellererLynn, K. 2004. Rocky Mountain National Park. Geologic Resource Evaluation Report. NPS D307, September 2004. Online at: (visit link)
Rocky Mountain National Park. Online at: (visit link)
Rocky Mountain National Park was most helpful in the background discussion, aid in the choosing of sites, and review of this EarthCache. My thanks to the Park for allowing the placement of this EarthCache!
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