In this task, you will be presented with a question and you have to answer the question based on your knowledge. Your answers should be as short as possible.

Let me give you an example: Who were the builders of the mosque in Herat with fire temples ?
The answer to this example can be: Ghurids
Here is why: This is a good example. Ghurids were the builders of the mosque in Herat with fire temples.

OK. solve this:
What kind of rocks cover the structure of the crater that 's in Alaska ?
Answer:
Permanently frozen Pliocene and Pleistocene rock