paragraph: Like flowing water, flowing ice erodes the land. It also can deposit the material elsewhere. Glaciers cause erosion in two main ways: plucking and abrasion. Plucking is caused when sediments are picked up by a glacier. They freeze to the bottom of the glacier and are carried away by the flowing ice. Abrasion occurs when glaciers scrape over the Earths surface. The ice sheet acts like sandpaper. The ice contains sediments and rocks frozen in the ice. The rocks and sediment grind away as the glacier moves. They wear away rock. They may also leave scratches and grooves in them. Scientists use these grooves to learn about the direction the glacier has moved. 
question: What leaves scratches and grooves in rock?
answer: Rocks