A friend of mine and me tried going up Mill Creek and straight up one of the chutes that goes directly to the summit. We did it when there was snow (the creek bed and chute were totally covered but late enough in the season to be firm enough to walk on. After playing dodge ball up the chute with grape to baseball sized rock fall, About 500' below the summit I heard some large noises that could only mean something big was coming. We both retreated off the perfect crampon snow/ice of the chute to opposite rock cliffs on the sides. Sure enough a torrent of rock came bouncing down the biggest being the size of a refrigerator that took about six bounces as it flew past us (reminded me like we were in a pinball machine. Needless to say, we retreated after that (it was only getting warmer as the morning wore on and rock fall continued). The rock in the area is in such bad shape because Mill Creek is one of the branches of the San Andreas Fault. As you say, a canyon for drier weather but even then watch for rocks.