Thunder Canyon Cave is the most extensive known granite talus cave in San Diego and one that I had been curious about for a long time. So when I heard a trip was being led through the SoCal Grotto I jumped at the chance to get on a trip to check it out. A very early departure from Riverside had me the first to arrive at the meeting spot, a pleasant enough area of classic San Diego rolling granite hills that seemed to have good bouldering and climbing potential. I enjoyed the quiet apart from the birdsong as I packed my bag and others slowly arrived. In all there were ten of us, a solid group, and somewhat surprising most were about my age best I could tell. Everybody seemed very friendly. For a couple this would be their first vertical cave trip.
We set off along a meandering network of dirt bike trails towards the caves, the trip leaders thankfully having good memory of the cave's whereabouts- this would be an easy area to hide talus caves! It was already getting a little warm but we chatted as we cruised along, reaching the cave in a about an hour. The entrance was completely unassuming- just a particular space between several boulders in a field of thousands of other similar gaps. One of the trip leaders rigged the "top drop" entrance while the rest of us snacked and readied. I was quick to get in line to drop into the cave, craving its cool confines and curious to explore it. The drop was a little awkward the way it led down a chockstoned crack and over a bedrock fin but I was soon down.
The cave is no Millerton Cave (wet, active, intricately fluted and potholed, king of CA talus caves) but I was glad to see that the same water-polished granite walls were present throughout the cave. Occasionally water did flood spectacularly through this cave but the intermittent nature meant there were flowstone falls and rimstone dams in places, both better developed than in any other granite cave I've seen. It was clear there would be considerable time spent waiting for everyone so I convinced someone to give me a boost so I could climb the flowstone fall up into the uppermost portion of the cave. Here a nice 50ft stretch (Serpentine Passage) led to a more prolonged tight slot. I hung out at the top of the falls taking photos of the others coming down.
Once we were all down the drop and in the cave we moved as one group through the narrow passage, working our way over and under boulder chokestones and shimmying through narrow spaces. It was all very social with a person just in front and just behind at all times. Progress was slow but we eventually reached the middle entrance area (Bat Entrance), where one of the party decided to climb out. While that was happening I moved the rope down the cave and rigged the Cathedral Room pitch. This room was easily the most surprising and standout feature of the cave. Instead of the narrow slot passages a balcony-like hanging pothole led to a 50ft flowstone waterfall descending into a spacious room (frontispiece). I was the first down and waited a solid 35 minutes until the others began to descend, but it was easily the nicest and most comfortable place to wait. The temperature was so perfect in this cave as to not render a conscious thought about it.
In the Cathedral Room we all got a briefing of the rest of the cave. We'd have a handline down a vertical chimney, then another down a flowstone falls, then the infamous squeeze, which has been the site of at least major rescue. A lot of care went into sequencing everyone for the squeeze and passing through all the packs. Everyone got through the squeeze without much issue. I didn't even have to take my harness or helmet off so I'm not sure I'd really count it as a squeeze.
One of the group managed to drop their headlamp into the tighter bottom part of the crack, which sans helmet and harness now, gave me an opportunity to really test the squeeze. I was able to recover it without much effort. One final short climb and a bit of a breakdown maze brought us out the bottom entrance and into the warm sun. The plan was to hike back up to the top entrance with someone dropping into the middle entrance to de-rig the drop there. Nico and I instead volunteered to go back through the whole cave, de-rigging everything, and meeting at the top entrance. With some reluctance we were given the go ahead and went back into the cool confines of the cave. With just two of us we made excellent time, even including ascending the drops and de-rigging them. The ~500ft of cave which took the group over 4 hours to traverse took the two of us 25 minutes to return through. In fact the others beat us to the top entrance by only a couple of minutes. It was immensely satisfying being able to balance out the slow but social trip with a second exercise in speedy efficiency. The uphill walk to the car in the afternoon sun was slower, but eventually we all made it back. It was probably the slowest cave trip I've ever been on before, but I enjoyed seeing the cave and enjoyed meeting nine friendly people in the process.
I've been in at least a half dozen granite talus caves in California now and find them endless fascinating and enigmatic. The general idea is that they are deeply incised slot canyons that then have boulders collapse over the top to create an enclosed space. While this works for much of the length of the caves, all these caves have passages where the floor, walls, and ceiling are all in situ bedrock indicating that along some stretches water is capable of tunneling through the granitic rocks rather than just downcutting. For a few meters a stream could do this by drilling a bedrock pothole and then breaching its wall (and this does happen often), but for longer stretches a different mechanism seems to be at play. I'm not sure Thunder Canyon gave me any new insight, but these caves remain of interest. Thanks to the trip leader Jess and Nico for going back through the cave with me.
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