Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Mountain streams may erode waterfalls. As shown in Figure below , a waterfall forms where a stream flows from an area of harder to softer rock. The water erodes the softer rock faster than the harder rock. This causes the stream bed to drop down, like a step, creating a waterfall. As erosion continues, the waterfall gradually moves upstream. Answer: a waterfall
A:
When a stream flows from a hard to soft rock area, what is often formed?