The Twin Sisters are both massive outcrops of granite, however the "left sister" consists of 2.5 billion year-old granite whereas the "right sister" consists of 30 million year-old granite. The Twin Sisters are near the southern boundary of City of Rocks National Reserve (Strickand and others, [2008]).

The URL is: https://gotbooks.miracosta.edu/gonp/ciro/html2/cor00073.htm Last modified: 1/10/2011