You are looking at colluvium: angular or jagged rocks with rough edges.  When you see colluvium on a slope like Tempe Butte, you know the rocks have not traveled far from their source.  In this case the rock is andesite, and the longest these pieces of andesite colluvium might have moved downslope (mass wasting) is a trip from the top of Tempe Butte.  As rocks travel farther, through stream transport, the jagged corners get smoothed and rounded, and the rocks transported by streams are then called alluvium.  You will see alluvium on the other (north) side of Tempe Butte.