As glaciers move over bedrock, they erode the bedrock in a variety of ways. One way is by crescentic gouges. Crescentic gouges are thought to form (no one has actaully seen one form) when a large boulder being carried along in the ice comes into contact with the bedrock and stresses the bedrock in such a way as to cause a little piece to break off. The textbook (Glacial and Periglacial Geomorphology) shows the boulder in the lower right corner and a crescentic gouge in the upper right. For me, it is fun to be able to make these direct connections between what I see in textbooks and what I see in nature.