When basaltic lava flowed into the Grand Canyon a few hundreds of thousands of years ago, a few of the flows were so voluminous that they filled the old channel of the Colorado River. Here, near river mile 183, you can see where a former channel of the river was filled with lava. The river then cut a new canyon to the left of this - the modern channel that river runners ride through today! (Photo and caption by Wayne Ranney)
Interactive geologic map of Grand Canyon: http://rclark.github.io/grand-canyon-geology/
