Bull Valley Gorge is a slot canyon in the Navajo Sandstone. It is about 200 ft. deep at its deepest, and in places it is narrower at the top than the bottom. There are a few small downclimbs and a pool or two, but it can be traversed without technical equipment if you have good slickrock scrambling skills.
Park just before the "bridge" over the gorge. A clear trail begins northwest of the bridge. Follow the trail westerly along the rim of the gorge. You get a great view down into your destination; occasionally, there will be some exposure, but the trail is safe. At 0.4 miles, the trail will disappear into a flat bench of slickrock, below which the slot has become quite shallow.
You can either go upstream until it is easy to drop into the gorge, but you'll have to downclimb a 10 ft. dryfall that will be difficult to climb back up. Alternatively, at the edge of the slickrock area, look for a crack in the rim. This can be climbed down and up about 20 ft. As is always the rule, don't go down anything you cannot confidently climb back up.
Either way, just head downstream as far as you want. The only significant obstacle is a couple hundred yards downstream of the crack, where there are two short drops, which sometimes have small pools at the bottom. This can be very difficult when it is muddy, but there is a bolt and sometimes a rope that makes it easy to get down and up.
One of the highlights of the gorge is going under the bridge
Continue downstream until the slot opens up into a wider canyon (the best is about a total of one mile in the canyon, and still good for about another mile), or until you want to return. It is possible to continue to Sheep Creek on the Bull Valley Gorge Route
and form a loop with Willis Creek Narrows Trail
, but it is shortest to just return the way you came.
P.S. As with any slot canyon, disregard the elevation profile shown here, as the elevation data usually doesn't reach the true bottom. In fact, the entire slot canyon drops very gradually.
The bridge was formed in the 1940s by simply throwing boulders and logs into the narrowest gap until they stuck, then covering them with 30 ft. of fill. The truck fell in (backwards) in 1954, killing three local men.