Joints - Granite dominates this portion of McDowell Mountain Regional Park, and with granite comes jointing.  Joints are cracks in rock, not what college students sometimes think:

Compression, shearing, and temperature changes all act in different ways to create these joints.   Imagine a body of magma a kilometer or more across.  The magma cools very slowly to produce the granite rock.  That cooling, however, does not produce granite joints.  Extreme stresses come afterwards when regional pressures act to squeeze, twist, and pull apart the body of magma until it cracks — making the fractures or joints.  In the case of granite, the cracks tend to break granite into roughly cube-shaped blocks, no matter whether you are in the McDowell Mountains :

or Joshua Tree National Monument seen here:

In the Chiricahua Mountains of Southeastern Arizona, the cooling of volcanic rock also caused vertical joints:

No matter how the rock forms, it is always fractured into joints.  We start this field trip with a discussion of joints, because these weaknesses "set the table" for what you will see later.