Limestone sits beneath ramparts of pink monzonite granite, which are younger than the limestone is in geological age terms. The gigantic white block of Notch Peak sits at 9000 feet just to the right of center in the distance.

This is one of the neatest geological combinations I have ever seen. At the base of the House Range in remote western Utah, you can observe pink granitic monzonite interbedded with limestone. The main upper portions of the House Range are all limestone, thousands of feet of it, including the dramatic cliffs of Notch Peak. Those layered carbonate strata were shoved upwards in part by the pressure of intrusive monzonite plutons forming below them, although the main uplift of both rock types into towering mountain ranges was done by even larger tectonic forces.

An interface between limestone below and monzonite above. Monzonite is closely related to granite but has less quartz in it, under 5% according to geological definitions. The shrub is a Mormon tea, possibly Ephedra nevadensis.

What is so interesting about this is the vastly different geologic origins of the two rock types. Limestone is a sedimentary rock, laid down in water, over eons of time often spanning millions of years. Most limestone has a biotic origin, consisting of the calcium carbonate skeletons of aquatic organisms including algae, diatoms, molluscs, and corals. Meanwhile, granite and monzonite are igneous rocks which are molten beneath the surface of the earth’s crust, slowly cooling into crystalline forms including feldspar, plagioclase, and quartz. Neither granite nor monzonite are ever liquid at the surface of the earth’s crust, unlike other igneous rocks such as lava and basalt. It is only later uplift and erosion that exposes these igneous rocks, post-solidification.

Here you can see how when it was deeply buried and still liquid, the monzonite forced its way in between limestone layers, wedging them apart. Note the faulting that forms in the offset layers of limestone as they were sheared apart. So unusual to see these two enormously different rock types in direct contact!

Details of the limestone layers. Each layer, dark or light, is about 1/2 to 1 inch thick. I do not know why they are different colors, although I could guess that it might be differing water chemistry resulting in different organisms predominating at various times? Perhaps it’s seasonal in nature? Although the layers do seem a bit thick to be deposited in only one season….

Hiking along at the base of the west face of the ultra-dramatic Notch Peak a month ago, I was struck by the way there were fingers of monzonite interlaced with strata of limestone, in an embrace rarely seen given how greatly different these two rock types are. The monzonite was intruding into the limestone, forced in by great pressure and heat from millions of years ago. Thin layers of monzonite crept in between the layers of limestone, similar to how one might press flowers in the pages of a book, or bake a layered cake.

A Mormon tea shrub (Ephedra sp, possibly E. nevadensis) grows on the limestone beneath the monzonite cliffs upslope. The trees in the background are mostly single-leaf pinon pines and Utah junipers.

On top of it all and on nearby slopes is a surprising array of flora, including several cactus and yucca species that I was not expecting to see this far north. More on those later.   It’s a site worthy of future return visits for numerous reasons.

Both rock types form cliffs and steep faces, although they have different hardnesses and therefore differing weathering rates.

Like a layer cake, the two types of rock are interwoven by some sort of geological wizardry. This region of western Utah is under study to become formally protected as a wilderness area at some point in the future.