In order to transport grain from Hayden's mill, a rail line was constructed.  The construction crew used dynamite to blow up the cement-like calcrete and create this "cut".  In the railroad cut, the calcium carbonate has completely filled the pores in soils, enveloping angular andesite colluvium. 

The calcrete in the railroad exposure is more than 2 meters thick.  The only place in North America with a thicker accumulation is in Texas, in the Ogallala formation.  The calcium carbonate you see in these images took at least hundreds of thousands of years to accumulate, perhaps more than a million years. 

This means that the south side of Tempe Butte has been relatively unchanged for a very long period of time.  Bits of rock (colluvium) break off from the top of the mountain, move down the slope (mass wasting) to the bottom, and then are frozen in place by calcrete formation.  When you go around to the north side, you will see a very different story.