Large boulders known as glacial erratics, are evidence of past ice ages. These boulders are deposited by the constant downhill movement of the glacier and left exposed as the glacier recedes. This glacial erratic was found at Upper Ice Lake in the San Juan mountains of southern Colorado. The San Juans are a high altitude range of mountains that straddle the Continental Divide. This wide-open landscape, at 12,300, is well above timberline. This photograph was taken from Upper Ice Lake in the San Juan National Forest near Silverton, Colorado, USA.