Mechanically weathered rock is broken down into smaller (four word phrase)

Weathering because Weathering is the process where rock is dissolved, worn away or broken down into smaller and smaller pieces. There are mechanical, chemical and organic weatheringprocesses. Organic weathering happens when plants break up rocks with their growing roots or plant acids help dissolve rock.

 
Mechanically weathered rock is broken down into smaller pieces because of nature.

Yeah, but what are you trying to ask?