The surrounding area was radically modified with the construction of the lake and the dam. Fortunately, Pyramid Rock has remained untouched and lies across the creek. This rock was formed in 1933 when crews blasted away the hillside to make a cut for US 99. There was originally a bridge that carried US 99 across the creek at this point, but it was removed and replaced by fill in 1951 as a cost cutting measure. It is possible to go to the other side to have a closer look at Pyramid Rock, which is done by way of the contemporary bridge built by the State Water Project.
Pyramid Rock is comprised of shale, which is why it exists.
Shale is fairly tough to blast out and the road builders only wanted to blast out what they needed for the road. The ridge that was blasted through was roughly triangular, which caused the remaining portion on the left to be shaped like a pyramid, hence the name. If left alone, it may even outlast the pyramids of Egypt. It can be reached by exiting at SR-138 east and going for about one mile to the intersection with Gorman Post Rd. It is fairly short since it is now covered by the newer alignment of SR-138, but it still is worth having a look.