In the rocky hillocks of the Minnesota River valley between Montevideo and Morton, some of the oldest rocks on earth are exposed. The pink-and-black banded rock found there is a metamorphic rock called gneiss, specifically Morton Gneiss, formed when great and prolonged heat and pressure deep in the earth altered earlier rocks so intensely that the character and age of the original rocks were obscured. This metamorphism occurred about 3.6 billion years ago, probably during the creation of an early continental mass, as small pieces of continental crust collided and stuck together to form protocontinents, compressing their margins and producing the heat and pressure that caused the metamorphism. This process continued for billions of years, building larger continental masses like those seen today. Commercially, this rock is known as Rainbow or Tapestry granite, and gneisses have become popular building and monument stones throughout the world. North across the Redwood River, a twenty-three-meter bank of residual clay is all that remains of some gneiss that completely decomposed, a type of deep and intense decomposition characteristic of weathering in tropical climates. Such a climate existed here about 100 million years ago, when the North American continent, during its very slow drift across the surface of the earth, was located at subtropical latitudes of thirty degrees to sixty degrees north, reflecting the long-running global process of plate tectonics.