This is the inside of the culvert barrel.  If this was actually built about 1876 as expected, it is nice work indeed.  It seems clear that the rock used for construction was from Red Mountain, and probably came from higher up the south side of the mountain.  This natural material was very well suited for "stacking" as the bedding planes were very smooth.  A mason would have only had to dress the vertical faces and lay the stones on top of one another.  Small stones, such as the one on the right side foreground, would be used to keep everything level.