The peaks were created more than 20 million years ago during the Miocene Epoch of the Neogene Period, as volcanic plugs of magma which welled up and solidified inside softer rock which has since eroded away.
The peaks in order from Morro Bay to San Luis Obispo. including their height.
- "The Nine Sisters of San Luis Obispo County". Sierra Club San Luis Obispo. Retrieved 2009-06-25.