The above photo shows a close up of an andesite rock that was once inside the soil.  You see it at the surface, because of erosion of the overlying dirt and rock. 

This large angular andesite rock is breaking apart.  Look carefully, and you will see white lines going through the gray andesite.  The white lines are calcrete, a precipitation of calcium carbonate.  The precipitation of calcrete starts in a tiny crack that you can't even see with the naked eye.  Over time, more and more calcrete forms, and the crack widens. Eventually, a large block of rock is split into fragments as you see here. 

"Breccia" is the formal term given to a formation angular rocks. In the railroad cut by Hayden Mill, you can see lots of examples of the angular rocks (colluvium) cemented together by calcrete to form a breccia like this:

POP QUIZ FOR THE "A+" NATURAL GEOMORPHOLOGIST:  You encounter the rock below. It has a bunch of parallel fractures, like the andesite above.  Only the calcrete is missing.  What's going on?  This is a very difficult question. Click here for the answer.