GEOLOGY AS SEEN FROM THE PORCH
THE GHOST CITY INN, JEROME
I have spent many an hour sitting on the porch of the Ghost City Inn in Jerome, Arizona. It’s not only the great view from the porch that I enjoy but as a geologist it’s hearing the view whisper its geologic story. I really don’t hear the voices, even though it is the Ghost City Inn; but still the geologic story is out there.
Looking northeast from the porch you look across the expanse of the Verde Valley (about 17 miles). The valley is bordered on the far side by the southern edge of the Colorado Plateau. The Colorado Plateau makes up the northern portion of Arizona and also parts of Utah, Colorado and New Mexico. The plateau was uplifted (or raised up) approximately 20 million years ago. This uplift is what caused the Colorado River to erode the Grand Canyon. The steep cliffs that exist at the southern edge of the Colorado Plateau are called the Mogollon Rim, which also occurs north of Payson. On the Colorado Plateau, looking north-northeast from the porch, you can see the San Francisco Peaks. They are located just north of Flagstaff, Arizona. The San Francisco Peaks was an active volcano approximately 1 million years ago. The present form or profile of this volcano was created when the volcano either collapsed into the magma chamber below or erupted sideways just like Mt. St. Helens did in Washington State in 1980. Sitting on the porch, if you extend the left and right slopes of the San Francisco Peaks you can imagine what the volcano used to look like. The volcanic activity in the Flagstaff area is still active (geologically speaking) since Sunset Crater (located just east of the San Francisco Peaks) erupted just 900+ years ago. A blink of an eye to a geologist.
Looking again across the Verde Valley to the bedrock cliffs of the Colorado Plateau, you can see two prominent sedimentary rock layers: a reddish layer, which makes up the red rocks of Sedona, and a light tan layer above it. Sedimentary rocks form from the deposition of sediments (gravel, sand, silt, clay, etc.) on the earth’s surface or within bodies of water that become lithified or cemented together. The reddish rock layer is called the Supai Formation and it was deposited 285 to 315 million years ago when the environment of northern Arizona was a floodplain or delta located along the coast of the continent. The light tan rock layer above the Supai Formation is called the Coconino Sandstone. When this sand was deposited 275 million years ago, northern Arizona looked like the Sahara Desert. The sand in the Coconino Sandstone is dune sand. Up close to this rock layer you can see the cross-bedding of the shifting sand dunes. Below the porch in front of you, the walkway at the bottom of the wooden stairs is made of pieces of the Coconino Sandstone.
Getting back to the Verde Valley, this side of the valley on which Jerome is located, is formed by the Black Hills; the mountain range to the southwest of Jerome. The Black Hills were formed about 10 million years ago by uplift along the Verde Fault. The fault runs down the southwest edge of the Verde Valley. The creation of the Black Hills caused a basin to form (i.e. the Verde Valley) between the Black Hills and the Colorado Plateau in which an immense lake subsequently formed. This lake existed in the Verde valley for approximately 7 million years (10 to ~ 3 million years ago). With time, fine silt was deposited at the bottom of the lake and built up to a great thickness. These lake sediments contained a lot of calcium carbonate or lime. After awhile (once again geologically speaking), the lake dried up but the Verde River remained. This left the lake sediments behind which turned into the rock: limestone (another type of sedimentary rock). This is the light colored rock that exists throughout the valley floor. The cement plant, that can be seen to the northeast of the porch on the valley floor below Jerome, uses this limestone to make cement. Since limestone can be dissolved by groundwater, caves and sinkholes (collapsed caves) exist within the Verde Valley. One of the more well known sinkholes is Montezuma’s Well which is located near McQuireville just north of Camp Verde east of Highway I-17. A large spring exists within the Montezuma’s Well and in the past the spring overflowed the sinkhole depositing travertine (composed of calcium carbonate) around the rim. A crack developed in one side of Montezuma’s Well through which the spring water escaped and emptied into Wet Beaver Creek. The Native Americans living in this area in the past built canals from the water escaping Montezuma’s Well to irrigate their crops. The spring water (that contains a lot of calcium carbonate) over time coated the canals with travertine preserving them to this day. In this same area, foot prints of saber-toothed tigers, mastodons and camels that roamed this area a few million years ago have been found.
Highway I-17 heading north of Camp Verde is a gentle slope compared to the steep cliffs of the valley wall across the valley as seen from the porch. This gentle slope that the highway travels on was not created by ADOT, but instead by a lava flow that erupted on the Colorado Plateau. The lava flow flowed down over the edge of the Colorado Plateau into the Verde Valley creating a ramp of basalt on which the highway was constructed.
The Town of Jerome owes its existence to copper ore that was exposed in this area due to the uplift along the Verde Fault. This ore deposit was created 1 to 2 billion years ago, when off the western coast of the then smaller North American continent, an island chain existed that was formed by volcanic activity (such as like the islands of Japan). Volcanic vents that erupted under the ocean near these islands deposited minerals around them. Today, these volcanic vents exist elsewhere under the oceans and are referred to as “black smokers”. The “black smoke” is just highly mineralized fluids coming from the volcanic vents. Around these vents, scientists have discovered life forms that exist on the ocean floor. Crabs, shellfish, etc. exist in a narrow zone between the frigid ocean water and the boiling water from the volcanic vents. These life forms are unique in respects to the other life forms on earth. Most life on earth gets its life energy from the sun, but these life forms existing near the volcanic vents get their life energy from the earth itself. Getting back to the ore deposits of Jerome, the island chain mentioned above then collided with the continent and was basically plastered onto the western edge of the continent making the continent larger. Through time (geologic time that is) the mineral deposits were buried and more land was added on the western edge of North America. Only after the uplift along the Verde Fault were these mineral deposits brought back to the surface of the earth, where they were subsequently discovered.
So briefly, since I scarcely scratched the surface (pun intended), that is the geologic story that can be seen while sitting on the porch of the Ghost City Inn.
Joseph Karl Drosendahl
November 12, 2011