Directional glacial-scour features. Photomicrograph of small friction cracks. Yellowstone National Park, Wyoming. Circa 1968.

Album caption: History and Dynamics of Glaciation in the Northern Yellowstone National Park. Directional glacial-scour features. Photomicrograph of small friction cracks. Flow parallel to striations toward the left. Sample collected from summit area of Mount Washburn and coated with thin white film for photographic enhancement. Wyoming, ca. 1968. No index card. Published as Figure 7-C in U.S. Geological Survey. Professional Paper 729-F. 1979.