25 Banded Gneiss from Millard County’s Mineral Mountains

An elderly friend of mine use to say that, “growing old is for rocks and trees,” and it seems that Millard County has both: our bristlecone pines are the oldest life on earth and our Banded Gneiss, (pronounced as nice), is part of the sequence making up Utah’s oldest rocks. Those rocks appear to be common in Millard County’s subsurface strata but they are found at the surface at only one lone exposure in the northern Mineral Mountains.

It’s an hour long hike to reach them up near the top of that range and probably not worth your effort unless you realize their history. At first they don’t seem like much to look at. They are coarse-grained and appear as “layers of light-grey quartz and K-feldspar with interspersed dark layers of biotite, plagioclase, and quartz.” The banding indicates a possible sedimentary bedding; it is thought that they originated as oceanic islands which merged through tectonic movement with the old Laurentin craton, a Precambrian continent in the process of creation; that craton eventually became North America. Metamorphism then took effect: The rocks were deeply buried, resulting in composition and structural change from the subsequent heat and pressure.

Metamorphism is expected in all the oldest rocks; without burial, along with the heat and pressure, how else could they survive the ravages of erosion over such long periods of time? There, with many feet of more recent rock above them, they lay protected as each geologic age followed in succession. Oceans, mountains, valleys, and plains all came and went above them – not once but many times; rivers and lakes formed and then disappeared; ice sheets advanced, later followed by tropical jungles, and still later by arid deserts; and through all this they remained buried. Born in the early Proterozoic Eon, they lay there through the remainder of the Precambrian as Earth experienced the first stirrings of primitive life; the coming and going of multi-cellular animals and plants; the first life on land; the vast Carboniferous forests which became our coal beds; the arrival of the dinosaurs and the age of reptiles to be followed later by the mammals. Through all this, some 1.75 billion years, they lay, waiting their eventual return to the surface. Then in recent times, just a few million years ago, the mountains we are familiar with were pushed up; the Banded Gneiss again saw the light of day. Here for a time; they too will eventually go away, destroyed by erosion. That is the nature of geologic history.

Some information for this essay came from Geology of Millard County, Utah by Lehi F. Hintze and Fitzhugh D. Davis, Utah Geological Survey, 2003 and some from The Changing Earth, Exploring Geology and Evolution, by James S. Monroe & Reed Wicander, Wadsworth Publishing Company, 1997.