What is this geological formation in the Santa Cruz mountains that looks like a cave interior?
Well given that travertine springs trail is right next door, in castle rocks state park I am going to guess travertine. Which is a calcium carbonate mineral deposited from geothermal springs, hot ground water brings up the dissolved travertine which is deposited as the hot water evaporates, but can also slowly be dissolved and redeposited by rain. Thy can form very quickly, on the span of years or decades. There are many travertine geothermal springs in the area. If you want more information try this link for a start.
https://en.wikipedia.org/wiki/Travertine_Hot_Springs