Here we have some nice little glacial striations exposed in the Grinnell Glacier cirque in Glacier National Park, Montana. These grooves were carved by pebbles and other clasts within the glacial ice as it flowed over this outcrop of the Mesoproterozoic Helena Formation (part of the Belt Supergroup). Perhaps some of the same pebbles you see in this photo were responsible for acting as carving tools -- though the 'hand' that wielded them, Grinnell Glacier itself, melted away from this point sometime since 1939.

Also of interest to me in this photo is the lingering stain of water around the joint set in the upper right. I'm fascinated at the interplay between physical and chemical weathering, and seeing stuff like this emphasizes how even a simple hairline fracture can help funnel water, with all its destructive effects, deeper into the heart of an outcrop. Weathering is focused on these areas, and in another century this outcrop may look quite different.

Labels: glacial landforms, glaciation, joints, montana, national parks, pleistocene, proterozoic, sediment, snow, weathering