The rock on which you’re standing is a composite of quartz, feldspar, biotite, and amphibole. It shaped greater than three-and-a-half billion years in the past when molten materials cooled to type granite deep within the stomach of the Earth.* Later, this granite was caught within the vice of plate tectonics, re-melting it in order that pink feldspar may wind its manner into the material. Step by step it hardened solely to warmth up once more, including nonetheless extra colours to the swirl. This was about 800 million years in the past (Ma). Then, about 100 Ma, it was rudely squeezed to the floor. The dinosaurs proliferated, the dinosaurs died. Newly assured mammals pawed on the rock. Then, ice. Glaciers flowed backwards and forwards at intervals throughout the long run northern states, protecting the gneiss in a thick glacial until. Lastly, about 12,000 years in the past, Lake Agassiz burst, unleashing an enormous bulldozer referred to as the Glacial River Warren. This reduce a deep gorge by what’s now the Minnesota River Valley, eradicating the until apron, exposing the gneiss, and making these damned potholes.
[* Individual zircon crystals from the gneiss have been dated to 3,524 ± 9 Ma. This makes it not only the oldest intact surface rock in the United States, but one of the oldest rocks on the planet. The oldest intact surface rocks on the planet are closer to 3.8 billion years old, although some have claimed older ages for rocks (e.g.) in the Canadian Shield.]