paragraph: While glaciers erode the landscape, they also deposit materials. Glaciers deposit their sediment when they melt. They drop and leave behind whatever was once frozen in their ice. Its usually a mixture of particles and rocks. It can be of all sizes, called glacial till. Water from the melting ice may form lakes or other water features. Figure Moraine is sediment deposited by a glacier. A ground moraine is a thick layer of sediments left behind by a retreating glacier. An end moraine is a low ridge of sediments deposited at the end of the glacier. It marks the greatest distance the glacier advanced. A drumlin is a long, low hill of sediments deposited by a glacier. Drumlins often occur in groups. These groups are called drumlin fields. The narrow end of each drumlin points in the direction of an advancing glacier. An esker is a winding ridge of sand deposited by a stream of meltwater. Such streams flow underneath a retreating glacier. A kettle lake occurs where a chunk of ice melt as they are left behind as a glacier retreats. When the huge chuck of ice melts it leaves a depression. 
question: What can be deducted from the decreasing width of the drumlins groups?
answer: The narrow ends mark the direction of advance of glacier