Mount Calavera, Carlsbad, California
Last eruption about 29 million year ago, of this volcano all that remains today is the plug. The plug is what is left after a volcano becomes extinct and the hard material that came through the volcano's vent remained after the softer outer materials eroded away. This Volcano is one of the smallest in North America.
The mount has a quarry in its west side that was used from the early 1900's to later 1930's, for gravel. It gives the mountain a distinctive appearance when seen from nearby roads. The excavated pit exposes the inner throat of the now dead volcano. Look closely and you can see examples of columnar jointing.
There is another example of a volcanic plug in Jacumba, right off of Hwy 8 can be spotted Round Mountain.
Nearby to the quarry can be found a small cave, I have heard it call an exploratory mine, from what I see of the size of it and the entrance supports it almost looks like storage, possibly for explosives. You can see where the entrance was sealed off, but bypass later by digging around the blocked off entry.