Central Highlands Region

Figure 6: Permeability ranges for aquifer materials

The southern boundary of the Colorado Plateau is the Mogollon Rim, a steep ridge formed by erosion after the plateau was uplifted.  Large volcanoes, such as the San Francisco Peaks, are present along the Mogollon Rim bordering this Central Highlands Region or Transition Zone.  This zone cuts across central Arizona, (see Figure 4) separating the Basin and Range Province from the Colorado Plateau, and exhibits geologic characteristics intermediate between the two.  In addition to the volcanoes along the northern margin, it contains mountainous regions (highlands) cut by major canyons and valleys filled by unconsolidated sediments such as in the Verde Valley. 

The amount of water produced by wells developed in these valleys will vary depending on the grain-size of the aquifer material—fine-grained silts and clays will yield less water than the more porous coarse-grained sands and gravels.  Wells in the dense fractured volcanic basalt rocks will also vary in yield depending on the number of water-bearing fractures intercepted by the well borehole and permeability (see Figure 3 as well as Figure 6).