Black Mountain dominates our twin communities as it rises some 1000 feet above the high desert floor. The mountain’s most striking geological feature is its "split personality;" a reference to the marked contrast between the rock units of its east and west sides. The division is especially striking when you approach the 3,398-foot mountain from the south. The east side of the mountain, a golden-brown weathered granite, stands out in stark contrast to the gray-black slate and phyllite rocks that distinguish the western side, and give Black Mountain its name.
How did Black Mountain’s split personality come about?
The story begins between 1.7 and 1.8 billion years ago, when a mix of sedimentary and volcanic rocks were deposited, most likely in an ancient sea. At this time our area was actually located south of the Earth’s equator. Continuing deposition and deep burial within the Earth over a period of millions of years led to metamorphism: mineral changes caused by intense heat and pressure. During this time Earth’s tectonic plates carried North America toward its present position, well north of the equator, and the siltstone and sandstone turned into what geologists call a meta-argillite-phyllite complex, a varied rock formation that includes slate, phyllite and schist.
The best view of the dark-colored phyllite and slate is from Cave Creek Road where it curves around the west and north sides of the mountain. Depending on the time of day and your vantage point, the slate takes on different shades from shiny gray to black. These rocks are similar in age to those at the bottom of the Grand Canyon, and among the oldest exposed rock formations in Arizona. Tiny mica crystals in the metamorphic rocks sometimes reflect the brilliant desert sunlight of late afternoon.
The granite side of Black Mountain formed much later than the phyllite and schist side, approximately 1.4 billion years ago, as hot liquid magma cooled slowly and solidified deep inside the Earth. The cooling process occurred at a depth below the meta-argillite-phyllite complex, but forces deep within the Earth pushed the granite up until it lay alongside the meta-argillite-phyllite complex, which is how the two distinctively different rock formations came to rest side by side. Geologists call Black Mountain’s granite Camelback Granite because it is part of a formation that extends south to Phoenix’s Camelback Mountain. Its crystalline texture features pale pink feldspar, white feldspar, clear to gray quartz, and a black mica mineral called biotite. These individual minerals are quite easy to distinguish when you see a fresh, gray-pink exposure along a road cut. More than 12 million years of wind, rain, frost and fracturing split and molded the granite into the hundreds of monolithic boulders that you see hugging Black Mountain’s slopes like tumbled ancient ruins. The best views of the boulders are from the east side of Carefree Highway and the south end of Tom Darlington Road. The massive beehive-like boulders on the east side of Tom Darlington Road form the magnificent backdrop to Carefree’s Boulders Resort.
Occasionally, people speculate that Black Mountain may be a volcano. Indeed, the rounded pyramid shape is typical of many volcanoes. But the rocks are all wrong. Volcanic rocks would be uniformly red or gray-black without the fine layering evident in rocks on the western side. Furthermore, crystalline volcanic rocks would not display the large crystals that are so visible in the granite rocks on the Carefree side.
There are a number of pits and tunnels where late nineteenth-century prospectors hunted for the gold that led early pioneers and settlers to our area. The Mormon Girl Mine along the southwestern flank actually provided a modest income over a period of years roughly a century ago. But the gold was sparse, as is groundwater on Black Mountain. Near-vertical fissures on the west, and nearly solid granite on the east, hold very little water. However, a small spring near the source of Andorra Wash on the Cave Creek-Carefree boundary flows occasionally, when there’s enough rainfall.
As you gaze upon Black Mountain, think about its intriguing geologic contrasts. One side had its origins in an ancient ocean bed, where clay and mud were deposited nearly two billion years ago. The other side was volcanic in origin. One side formed through heat and pressure. The other side formed through cooling of magma. Think of the billions of years of weathering and erosion that led to the magnificent landscape we call Black Mountain. And think of the importance of preserving this unique geological legacy for generations to come.
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