Geologic Cross Section of Montezuma Well and Groundwater Flow
Graphic by Trista Thornberry-Ehrlich (Colorado State University) after Konieczki and Leake (1997, figure 15) and Johnson et al. (2012a, figure 9) with information from Lange (1957) and Lenihan (2011).
A Sinkhole in a Desert
Montezuma Well is a limestone sinkhole, continuously fed with water from an underground spring. Water that falls from rain or snow on the nearby Mogollon Rim trickles down through the rock until it reaches the spring. The pressure from the vent then pushes the water up to the surface. The water takes as long as 10,000 years to travel from the Rim to the Well!
Montezuma Well is a near constant aquatic environment. The Well formed when the limestone collapsed sometime between 12 and 15 thousand years ago. Water is fed into the Well through several vents at the bottom, and exits the Well through a swallet and cave system, with the outlet draining into a prehistoric irrigation canal. Both the water level and temperature are nearly constant throughout the year, with the first fluctuating by only 16 cm and the latter averaging 70 degrees F (Blinn 2008). The water also has a stable pH because of high alkalinity and an excessive amount of dissolved carbon dioxide, which enters the well from the bottom vents in concentrations in excess of 500 mg per liter, more than 100 times higher than normal (Blinn 2008). These extreme chemical conditions mean that no fish can survive in the Well, and that organisms living in the water have had to adapt in order to survive.
It can be common for fossils to be present in limestone formations and visitors can see one of those fossils at Montezuma Well near the outlet trail today! Fossilized plants and other organisms found in limestone around this area can help scientists learn about life in the Verde Valley millions of years ago.
Geologic Cross-Section Between Grand Canyon and Verde Valley
Graphic by Trista Thornberry-Ehrlich (Colorado State University) after Blasch et al.
Long Before Humanity, Powerful Forces Shaped This Land
The following timeline makes a very long story short:
Earth forms about 4.6 billion years ago.
About 1.76 billion to 1.74 billion years ago, during the Early Proterozoic Era, plutonic rocks intrude below Earth’s surface and volcanic rocks erupt onto it.
During the Paleozoic Era (541.0 million–251.9 million years ago), sedimentary rocks of the Cambrian, Devonian, Mississippian, Pennsylvanian, and Permian Periods accumulate along ancient shorelines and in shallow seas and deeper marine basins. In the Montezuma Castle region, the oldest unit in the Paleozoic sequence is the Cambrian Tapeats Sandstone; the youngest is the Permian Kaibab Limestone. Exposures of Paleozoic rocks crop out in the tributary canyons that cut the Mogollon Rim, including the narrow canyon near the headwaters of Wet Beaver Creek.
In the region, Basin and Range extension begins about 15 million years ago (Shafiqullah et al. 1980).
The Hickey Formation (sedimentary and volcanic rocks), accumulates and erupts between 14.5 million and 11 million years ago (McKee and Elson 1980). The Black Hills had not yet uplifted when basalt flows of the Hickey Formation spread across the landscape.
At about 14 million years ago, the Black Hills are rising as the Verde basin is dropping down along the Verde fault zone.
The Verde Formation accumulates in the down-dropped Verde basin between about 8.5 million and 2.5 million years ago.
Timing of the initial collapse of Montezuma Well is unknown but estimated to have taken place between about 5.5 million and 13,300 years ago. The well existed as a shallow pool starting about 9,000 years ago. Water levels began to rise steadily after about 1,500 years ago.
About 2.5 million years ago, the Verde River begins to incise into the Verde Formation, creating today’s Verde Valley. Incision by tributaries also began at this time. Terrace gravel and alluvium record the evolution of the Verde River valley and tributary drainages.
Modern river and tributary-stream deposits mark active channels. These deposits record ongoing geomorphic changes chiefly caused by flooding.
The exact age of Montezuma Well is unknown but is between about 5.5 million and 13,300 years old. Four pieces of evidence help to constrain the well’s age:
First, the well must be younger than the Verde Formation - travertine, which is present, as reported by DeWitt et al. (2008). The Verde Formation travertine dates from the Pliocene Epoch (5.5 million to 2.6 million years ago).
Second, the well must be younger than the basalt dike underlying the well, which the groundwater-flow model (Johnson et al. 2012a, 2012b, the figure above), aided in the well’s formation. DeWitt et al (2008) reported an age of 6.2 million to 5.5 million years old for this basalt.
Third, the well must be older than the water it contains; Johnson et al. (2012b) provided an age of between 13,300 and 5,400 years old for the well’s water.
Fourth, the well must be older than the diatoms it contains; Blinn et al. (1994) collected a record from the past 11,000 years. According to Lange (1957), following the initial collapse of the well, which was not of the magnitude of the present well but probably a good proportion of it, the pool was shallow with a water level that stood at least 4 m (14 ft) higher than at present. This water level was required in order that the adjoining cave was submerged so that silt could be deposited in the cave and the existing speleogens (bedrock features that stand out in relief on the walls, ceiling, or floor of a cave) could form. Studies by Davis and Shafer (1992) and Blinn et al. (1994) suggest the following timeline for the development of Montezuma Well:
More than 9,000 years ago: Montezuma Well contained shallow water of constant depth underground.
9,000–5,000 years ago: Water levels underwent substantial fluctuations, and sediments were occasionally exposed to the air.
After 4,000 years ago: Collapse in the travertine structure of the well resulted in a change in calcite deposition. This was the creation of the sinkhole formation.
After 3,000 years ago: Increased surface erosion occurred from steep slopes surrounding the Montezuma Well pool.
After 1,500 years ago: Water levels rose steadily
Petroglyphs at V-Bar-V Ranch and desert varnish
Chanel Wheeler CC BY-SA 2.0
Desert Varnish
Desert varnish can be found everywhere throughout Arizona but what exactly is it?
Desert varnish is the thin red-to-black coating found on exposed rock surfaces in arid regions. Varnish is composed of clay minerals, oxides and hydroxides of manganese and/or iron, as well as other particles such as sand grains and trace elements. The most distinctive elements are manganese (Mn) and iron (Fe).
Bacteria take manganese out of the environment, oxidize it, and cement it onto rock surfaces. In the process, clay and other particles also become cemented onto the rock. These bacteria microorganisms live on most rock surfaces.
The color of rock varnish depends on the relative amounts of manganese and iron in it: manganese-rich varnishes are black; iron-rich varnishes are red or orange; varnishes with similar amounts of manganese and iron are some shade of brown. Varnish surfaces tend to be shiny when the varnish is smooth and rich in manganese.
A complete coat of manganese-rich desert varnish takes thousands of years, so it is rarely found on easily eroded surfaces. A change to more acidic conditions (such as acid rain) can erode rock varnish. Lichens can also chemically erode rock varnish, as can visitors who scratch graffiti into it.
