In Colonial days fences were often made without vertical
posts, because they did not have posthole diggers. The logs
were split, the laid at an angle of almost 90 degrees to
each other with ends overlapping enough to be stable.
The zig-zag made them quite stable. Here's pictures of a
modern version, which, however does use vertical posts.
Remember Abraham Lincoln as a young man worked as a rail-splitter.
http://home.comcast.net/~matancero/BigWormFence1.html