Glacial Abrasion
Living in the Great American Corn Desert, a landscape largely dominated by glacial deposition, displays of how the continental ice sheets were able to erode bedrock may be hard to find or not always be obvious. At Mounds State Park, a place famous for how the Adena-Hopewell people altered the landscape some 2,000 years ago, there are a few Ice Age lessons to be learned about glacial erosion.
The coordinates take you to the boulders, labeled 'A' and 'B' in Figure 1. 
Figure 1.
These two rocks have a story to tell.......
Striations, polish, and grooves:
Imagine......if you took an ice cube from your freezer and rubbed it REALLY hard on your sidewalk, how much damage do you think you would do to the concrete? Go ahead, try it.....Ok, so the sidewalk wins. Glacial ice is basically the same stuff as your ice cube, so how do we turn this wimpy material into a rock-grinding machine?
Now imagine......if you took as sharp rock and rubbed it REALLY hard against your neighbor's sidewalk (Remember: I'm NOT really telling you to do this, so don't blame me if you get in trouble!), how much damage to the concrete do you think you would do now? Yes, you would leave a scratch. Rocks that have been incorporated into the base of the glacier get dragged along the ground grinding the bedrock like a giant piece of sandpaper (Figure 2).
Figure 2.
This 'glacial sandpaper' cuts grooves and scratches into the rock, as well as flattens and polishes one side of the rock. The 'grit' of this glacial sandpaper, the rocks being dragged against the ground, also have grooves, polish and striations worn into them from contact with the bedrock surface, and even the 'grit' grinding against each other (boulder against boulder). The wet surface of this granite helps show the faint glacial striations (Figure 3.)
Figure 3, (photo by Stinky)
Because of water that melts and refreezes at the base of a glacier, rocks are sometimes flipped or rotated so that more than one side of the rock will be scratched and polished. One of these rocks (A or B) has striations on more than one side, indicating that it rolled and flipped as it was dragged against the ground by the glacier.
Crescentic Marks
In very hard, brittle rocks like quartzite (both A and B are quartzite). There may be curved (crescentic) fractures or curved chips of rock missing (crescentic gouge) that accompany the striations and polish. These are caused by other rocks being pressed against 'A' and 'B' under great pressure. Both rocks display crescentic marks, but they are better preserved on 'A'. Figure 4 shows a crescentic gouge on rock 'A'.
Figure 4, photo by Stinky.
Plucking or Quarrying
Unlike abrasion, this is a process whereby large chunks of bedrock are broken off and incorporated into the moving ice. In general, you need two things for this to work: 1) melt water at the base of the ice; 2) fractures in the rock. If the glacier encounters a bedrock hill or protrusion, there will be a great deal of pressure and friction on the side facing up-stream; enough to cause some of the ice to melt. The water would move from the high-pressure zone to a low-pressure zone i.e. the back side of the hill where it would enter cracks in the rock, freeze, and open the fractures little by little until the chunk of rock broke free to be carried away by the ice. Figure 5 shows what this might look like.
Logging Requirements
Examine rocks 'A' and 'B' and email (do not post) the answers to the following questions:
1) On rock 'A', are there other crescentic gouges other than the one pictured above?
2) If 'yes' to question one, are the orientations the same or different? ('different' would mean different directions of movement across the surface of the rock).
3) Which of the two rocks (A or B) show evidence of being rolled or flipped as it was dragged underneath the glacier?
4) How many different sets of striations can you see? Note: don't try to roll the rock over and look at the bottom. I didn't---you shouldn't).
This is the Honor System, you do not have to post a picture of yourself unless you want to.
