Alibates Flint Quarries National Monument - http://www.nps.gov/alfl

For thousands of years, people came to the red bluffs above the Canadian River for flint, vital to their existence. Demand for the high quality, rainbow-hued flint is reflected in the distribution of Alibates Flint through the Great Plains and beyond. Today this area is protected by the National Park Service and is the only National Monument in Texas. The monument can only be viewed by ranger-led guided tours. Tours are offered by reservation only.