How do we know that earth's magnetic field has undergone reversals in the past?

We know that the poles had changed positions in the past, because of the arrangement of the crystals in the rocks made out of metal. The crystals of the metals act like the needle of the compass when the rock is cooling off and forming, they turn into the direction of the strongest magnetic field on the Earth, which is the North Pole. So by studying the rocks that are made out of metals and see the crystals arrangement and also estimate the age of the rocks, the scientist can precisely tell how has the North Pole been moving throughout time.

3.1439 im dont think its right but im pretty sure it is

Answerhave flowing water either episodically or during only a portion of the year;