What is a Waterfall?

How is a waterfall formed questions

1. What are the two different types of rocks needed for a waterfall to form? 
2.  What does the water form when it falls off the top part of the waterfall when it hits the bottom?
3. What does the plunge pool do under the hard rock as it continues to fall? 
4. When the water undercuts the rock which way is the river going?
5. What happens to the hard rock as the water flows over it?
6. When the water goes upstream what does it form?
7. Name one waterfall in Michigan.
8. Name one waterfall in New York.
9. Name one waterfall in Canada.
10. Name one waterfall in Africa.
11. Name one waterfall in Europe.
12. Name one waterfall in Asia.
13. Name one waterfall in Australia.
14. Name one waterfall in South America
15. Name one waterfall in Antarctica