The Deem Hills are composed mostly of ancient Precambrian granite and other Tertiary age volcanic rocks, dating from nearly 2 billion years ago. During this time, rock fractures appeared where dark lava called basalt flowed and gave Deem Hills its characteristic black rock formations. In fact, much of this hilly area west of I-17 is comprised of the same basalt rock. A new trail in the Deem Hills trail system is named Basalt Trail. The name Deem Hills can be traced back to 1922. At that time, Dennis and Carl Deem began homesteading 160 acres southwest of the hills.
Deem Hills makes up almost 1,000 acres of Arizona Upland vegetation. The area showcases a variety of vegetation such as saguaros, barrel cacti, brittlebush, hackberry, ocotillo, palo verde trees, creosote, globe mallow, and buckhorn cholla; just to name a few.