The Late Cretaceous and Early Tertiary periods were geologically eventful in the West. The Rocky Mountains, which were uplifted about 50 to 100 million years ago, extend from southern Colorado northwest to the Canadian border. Their rocks and topography are diverse and highly complex. Many of the individual ranges that make up the Rocky Mountains appear on the map as variously shaped bull's-eyes surrounding a red-hued center. Each crudely ringed pattern was created by the Tertiary erosion of Paleozoic and Mesozoic sedimentary rocks that once overlay and now surround a core of uplifted Precambrian granite.