Devils Tower, Wyoming. In 1906 Teddy Roosevelt established this as the first United States National Monument. The tower is a 40.5 million year old phonolite porphyry intrusion, with prominent columnar joints formed during cooling. The softer surrounding sedimentary rock has been eroded away. Years ago I climbed the tower, but was forced off by freezing rain.
All images copyright © 2016 F.W. Vollmer and may not be used without permission.