The "Twin Sisters" name - a label given to the City of Rocks' pair of historic high-rises nearly 150 years ago that has since stuck - is actually incorrect.
The two granite pillars - each reaching some 60 stories high - are more than a billion years apart from being "twins" and are the products of separate rock formations. However, the duo overshadowing the southern end of the City of Rocks National Reserve illustrates some of the area's geological significance.The City of Rocks is the product of simple erosion, with the eroded, jointed granitic rock - light-colored, course-grained rock with salt-and-pepper-like specks - accounting for the blocky, scenic rims and predominant vertical stature.
The City of Rocks features the Green Creek Complex, a Precambrian rock formation dating 2.5 billion years and considered the oldest rock in the western continental United States. Some 30 million years ago, the 40-square-mile Almo Pluton - a softer dome of granitic rock or batholith - formed and cooled deep within the Earth's crust before it was extruded through the older rock.
Many of the formations - the taller granitic "islands" - seen today are the remainders of the Pluton's extrusion.
Each of the Twin Sisters represents the two different rock formations - the darker "sister" on the south is the older 2.5-billion-year-old monolith. And the lighter "twin" on the north protruded some 30 million years ago.
Water seepage and frost wedging are just part of the natural weathering and hardening that have shaped the rocks over the centuries, with moisture depositing minerals as it passes over outer layers.