If there were just one word in the entire English language I would choose to express a sense of the rocks in Snow Canyon State Park in southwest Utah, it would have to be contrast.
This is not a huge park – Snow Canyon itself is only about five miles long, with elevations ranging from 3000 ft at its mouth to 5024 ft at a peak just northwest of the canyon – but the visual effect is stunning. Overlying the lower canyon slopes of thinly bedded, rusty-red horizontal strata of the Kayenta Formation, what catches our eye throughout the park are the massively cross-bedded, white and orange-red cliffs of the Navajo Sandstone.
Particularly striking, too, is the interfingering of these variegated orange-red and white rocks. However, the contrast does not end there. Fields of black basalt boulders, jagged relicts of relatively recent lava flows extruded from higher elevations, cascade down through joints and fractures in the rounded multi-hued Sandstone.
I came out to Snow Canyon for a leisurely New Year’s Day hike with friends and found myself intrigued by this interfingering of red and white. Why does this occur? Is the whitening of the Navajo Sandstone strictly a surface feature or is it found deep within the rock?
The minerals contained in the cement which hold the predominantly quartz sand grains together are what give the Sandstone its color – variations in aluminum, sodium and especially iron determine if the Navajo Sandstone is red (higher percentage of minerals) or white (lower percentage of minerals).
The red cementing in Snow Canyon does color the entire rock and is not simply a surface stain; whitening occurs after lithification, the processes by which loose sediments are, over geologic time, converted to rock. The loss of red color is caused by fluid interactions – either the cementing minerals are dissolved from between the sand grains, or chemical reactions change the oxidation state of the cementing minerals, primarily the iron.
One possible explanation for the occurrence of these interfingering rainbows of red and white Sandstone can be found in fluids associated with the nearby Pine Valley Mountain laccolith, emplaced around 20 million years ago. Interestingly, the white Navajo Sandstone extends all the way from the base of the mountain to the west wall of Snow Canyon. This distance is perhaps five or ten miles, depending on the route your crow flies.
Sprinkel, D., Chidsey, T., Anderson, P., eds, 2000, Geology of Utah's Parks and Monuments, Publication 28, Utah Geological Association, p. 478-494
A Whole Bunch of People, 2007, Interim Geologic Map of the St. George 30'x60' Quadrangle and the East Part of the Clover Mountains 30'x60' Quadrangle, Washington and Iron Counties, Utah, Open-File Report 478, Utah Geological Survey