During the Eocene, approximately 38 million years ago, large masses of magma rose up below the crust in southwestern Idaho, northwestern Utah and northeastern Nevada, possibly in connection with mountain building going on further north and east. The vast underlying fields of magma forced their way into the overlying ancient sedimentary layers and cooled to form plutons of granitic rock.

Later, Basin and Range lifting lifted everything, plutons and sedimentary rock, to form mountains and valleys. Here, the Grouse Creek mountains formed approximately 13 million years ago. Erosion has since removed the old sedimentary rock and exposed the granite plutons to the surface.

From my layman's perspective, this particular granite seems to erode quickly, forming lots of amazing forms. I could easily break the edges of rocks with my fingers.