Given the task definition and input, reply with output. You need to read the given passage and construct a question about the information present in the passage. The question should be answerable from the passage. Most importantly, only those questions are allowed whose answers cannot be determined from a single sentence. Do not generate questions that require only a single sentence to answer. Instead, try to combine two sentences and create a question from that combination. Do not create open-ended questions, i.e., the answers to them cannot be found precisely from the information provided in the passage.

Paragraph- Sent 1: While glaciers erode the landscape, they also deposit materials.
Sent 2: Glaciers deposit their sediment when they melt.
Sent 3: They drop and leave behind whatever was once frozen in their ice.
Sent 4: Its usually a mixture of particles and rocks.
Sent 5: It can be of all sizes, called glacial till.
Sent 6: Water from the melting ice may form lakes or other water features.
Sent 7: Figure Moraine is sediment deposited by a glacier.
Sent 8: A ground moraine is a thick layer of sediments left behind by a retreating glacier.
Sent 9: An end moraine is a low ridge of sediments deposited at the end of the glacier.
Sent 10: It marks the greatest distance the glacier advanced.
Sent 11: A drumlin is a long, low hill of sediments deposited by a glacier.
Sent 12: Drumlins often occur in groups.
Sent 13: These groups are called drumlin fields.
Sent 14: The narrow end of each drumlin points in the direction of an advancing glacier.
Sent 15: An esker is a winding ridge of sand deposited by a stream of meltwater.
Sent 16: Such streams flow underneath a retreating glacier.
Sent 17: A kettle lake occurs where a chunk of ice melt as they are left behind as a glacier retreats.
Sent 18: When the huge chuck of ice melts it leaves a depression.

What are drumlin fields?