As far as I know we had 5 ice ages I know what caused them but how do we know we had those. (some million or thousands years ago.)
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Ice ages are really tough on rocks. These things called glaciers really damage the ground they move on because they are big and heavy and the bottom is like super-rough sandpaper. They leave characteristic marks on the earth. When they go away, those marks are buried by erosion infill. Then many years later another glacier comes by, that must be a different Ice Age.
As geologists dig down through the layers, they have found several different “ice age” layers, dated to different times in the past. As to the why, we also have ice cores that tell us about atmospheric composition for a long time into the past, so the different concentrations also separate warmer from colder times.
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