The Nine Sisters are a chain of nine volcanic peaks and hills in SLO county. Land uses vary, and while some are closed to the public, others are popular hiking, climbing, running, and biking destinations.
They are known to geologists as the “Morro Rock–Islay Hill Complex.” Nine of the peaks or morros (the sisters) have formal names, but there are an additional five peaks in the chain (including the informally-named Terrace Hill). There is also one offshore and fully underwater (the Davidson Seamount).1
They formed something like 22.1 to 26.4 million years ago as active volcanos at a time when the area was still submerged in the Pacific Ocean; what remains of them today is mostly their cooled magma cores.2
They are composed of dacite rock. The First Presbyterian Church and the County Historical Museum in San Luis Obispo are partly constructed with rock quarried from these mountains.3 The Morro Bay breakwater was constructed with some of the more than one million tons of rock blasted from Morro Rock. Many other local buildings, walls, rail lines, and other features were constructed with rock blasted from the morros.4
See Also
- Peaks that pique: A guide to hiking and exploring SLO County’s Nine Sisters by Rhys Heyden New Times 21 January 2015
Pages tagged “nine sisters”
Notes
- Sharon Lewis Dickerson Mountains of Fire: San Luis Obispo County’s Famous Nine Sisters—A Chain of Ancient Volcanic Peaks EZ Nature Books (1990), preface
- Dickerson op. cit. pp. 18–19
- Dickerson op. cit. p. 21
- Dickerson op. cit. p. 29
