This stop focuses on the development of soil in a desert.  Over time, rainwater washes calcium carbonate (same composition as Tums antiacid!) down through a desert soil.  This process of dissolving and leaching, called eluviation, is illustrated here:

The next question is what happens to the calcium carbonate that washes out of the A-horizon?  In a wet climate, the calcium carbonate would be washed out of the soil and down to the groundwater.  In a desert climate, there isn't enough water percolating downward to wash all of the carbonate away, so a lot of the calcium carbonate iilluviates, or deposits in the B-horizon of the soil. 

Eluviation and illuviation goes on for as long as the ground surface is stable.  After a few thousand years, the undersides ("beards") of cobbles in soils get a white coating.  After tens of thousands of years, cobbles are completely coated all over.  After hundreds of thousands of years, the entire soil is plugged with calcium carbonate.  These "stages" of calcium carbonate accumulation are shown here:

The calcrete on the south side of Tempe Butte is among the thickest and best developed that you will see in the entire world.  Scroll back up to the top and click on the pictures to examine calcrete formed on the south side of Tempe Butte.