During the ice ages, glaciers moved down the valleys of the Fall River and the Big Thompson. Glaciers carved U-shaped cross sections into formerly narrow stream valleys and left natural dams of glacial debris at their terminus. Striated and grooved bedrock, glacial erratics, and ridges of debris along valley walls are some of the other calling cards of ice. The ebb and flow of glacial advance is controlled by astronomical phenomena: the eccentricity of the earth’s orbit, the tilt of the earth’s axis of rotation, and the precession of the equinoxes. These move in cycles that combine to produce glacial maxima about every 100,000 years. The last glacial maximum was about 21,000 years ago. Since that time, the earth has warmed, not steadily, but in fits and starts that reflect other complex but poorly understood processes. Today all of these natural processes are further affected by man’s activities. The class will examine classical glacial features in the Fall River and Big Thompson valleys and discuss the processes that formed them.