Pete Tillman
Estero Bluffs State Park, blufftop trail
My favorite hike in this park!
Pretty much every rock you see on the bluffs here (or inland, where there are exposures) is radiolarian chert, en.wikipedia.org/wiki/Radiolarite . This is one of the few competent rocks in this part of California. As such, it was widely used as road metal in the early days, though it is just so-so as gravel (falls apart with use). The good stuff is the Morro Rock (etc) dacite, a strong volcanic rock that was widely used for breakwaters, building stone and crushed stone. About 1/4 to 1/3 of Morro Rock itself was quarried and used, from 1889 to 1969. Morro Rock itself is now in a State Park and protected from further quarrying. I don't think that there are active quarries in any of the other 8 or so Morro volcanic necks known.
Estero Bluffs State Park, blufftop trail
My favorite hike in this park!
Pretty much every rock you see on the bluffs here (or inland, where there are exposures) is radiolarian chert, en.wikipedia.org/wiki/Radiolarite . This is one of the few competent rocks in this part of California. As such, it was widely used as road metal in the early days, though it is just so-so as gravel (falls apart with use). The good stuff is the Morro Rock (etc) dacite, a strong volcanic rock that was widely used for breakwaters, building stone and crushed stone. About 1/4 to 1/3 of Morro Rock itself was quarried and used, from 1889 to 1969. Morro Rock itself is now in a State Park and protected from further quarrying. I don't think that there are active quarries in any of the other 8 or so Morro volcanic necks known.