If the "books" wasn't a plural, it would be "his" but since it isn't singular, it would be "their"
The speaker is talking about the water. During winter, the water enters the cracks and freezes and expands. It breaks the stone and makes cracks and when the water melts the holes are bigger. What he is describing is mechanical weathering which is common in nature.
Cause its frickin english
Im only in middle school, but what about whats going on in todays would versus back when your parents were young??
