This is a stone arch that defines a culvert to allow a stream to pass under an old dirt road. I think the construction could date to the 1800's. Please note that the stones forming the arch (voussoirs) are not rectangles -- they are wider at the top than the bottom. OTOH, the stones above the voussoirs are solid rectangles and are laid horizontally in an ashlar pattern -- each stone spans the gap between the 2 stones below it. In addition, you can see the arch stones and the horizontal stones interlock. As an example note the small block to the right of the keystone. That serves to stabilize the structure. The course of the stream is paved with a narrow slot in the center. That was done to reduce the amount of water flowing against the sides where it might undermine the stonework. The Romans used that method in their water supply systems. The entire structure is probably a barrel arch -- I'll check that when there is less water in the stream and no snow on the ground. In the near future, I will expand on this topic by writing a tutorial on construction of stone walls, arches, bridges, and related structures. I may also create a 3D model for producing realistic images of stonework. The image was acquired by using a Logitech ClickSmart digital camera and enhanced by using Adobe Photoshop 7.0 -- and no, I didn't fall into the stream. Thanks for visiting. All comments appreciated.