Root Creek originates in the upper reaches of the gully to the northwest of Castle Dome and flows down through a series of pools and waterfalls into the forested canyon above the Soda Creek exit on Interstate 5. The pools are only visible if you look down the east side of the Dome from the summit, and are reached after a bit of cross-country travel along the creek at the end of the Root Creek trail. Other than snakes and poison oak, another thing to watch out for in this area are carpenter bees.
These bees are unique because they live in cavities in wood, like downed trees and stumps. One memorable (if that's the right word) experience I had with them was in the early 70's when my friends and I spent a lot of time in the Root Creek area hiking and rock climbing. We began to work our way up the canyon to spend the afternoon bathing in the pools and drinking beer. Moving from one side of the creek to the other to follow the path of least resistance (and least poison oak) we came to a huge tree that had fallen over the creek, and it's 3 or 4 foot diameter trunk formed a natural bridge to get to the other side. Most of the bark was gone, leaving a rather smooth and slippery surface to walk on because of intermittent spray from the creek. Just about the time that the three of us had reached the mid-point of the log, and were 10 feet or so above the creek and 20 or 30 feet from either end, a swarm of carpenter bees came swirling out of the log and began biting us. I don't remember many details of what happened in the next few moments, but it was a testament to our nimbleness and non-verbal communication skills that we managed to get off the log without falling into the creek, but the damage was done and we all had numerous painful welts for the next few days.
This part of the Park is great on hot summer days because there's no southern exposure, most of the trail is shady, often times you'll be serenaded by the sound of woodpeckers practicing their morse code, and there's lots of water in Root Creek to cool off in. For some reason zillions of tiny spiders enjoy it too, because if you're the first person on the trail in the morning you'll end up snagging lots of webs strung from one side to the other, and as you walk they stream away from you like slender Nepalese prayer flags.
If you continue up the canyon to the pool/waterfall area, you can work your way up to and around the base of Castle Dome. One interesting feature that you might notice, but probably shouldn't linger to admire, are the many star-shaped impressions on the ground at your feet. These were formed when rocks fell (or were "trundled") from the upper part of the Dome and crashed into the ground at a bezillion miles per hour, shattering into lots and lots of smaller rocks. However interesting this may seem, this might not be a good place to hang out, because unless the falling rocks strike the Dome on the way down (over 1000'), you won't get much of a warning before they land.