I have read about sod cutters in Fireside or Mother Earth or some document like that. The usually have to do with building Soddies (Sod homes). While the mechanical ones have blades that are driven (sometimes by engines or sometimes by ground contact traction wheels) I recall one that was a sled or box. It had a knife that protruded from the bottom of the sled at the depth the sod was to be cut. It was "U" shaped just like one of the descriptions in an article below and all the leading edges were sharpened. Weight was required to hold the sled down and I remember the suggestion "put your children on for the ride." The sled was hooked to a horse or two and dragged across a sod field. The sod could be picked up later, but there was a device that rolled the cut sod out of the trough and onto the ground beside. I guess it was easier to pick up that way. Keeping the knife in the ground seemed to be the hardest part.
There were plow-like handles attached to the sled and someone wouldl walk behind the device. I think that pushing down or lifting up on these handles helped to keep the knife in the ground too.
Below are two fairly non-informative articles I found on the internet.
The pioneer constructed his own sod cutter. My father, being a blacksmith by trade, built one and it was loaned far and wide; in fact it almost never was at home. The cutting blade was sharp on the front side and bent in a "U" shape, thereby cutting the sod 12" wide. The sides, or upright portions of the "U" were fastened to two runners of sled effect, that served a dual purpose, namely to control the depth, so as to cut it of uniform thickness and to hitch the power to. They pulled rather hard, took a darn good team to pull one, as the sod was usually cut about 4" thick. It being of uniform width and thickness, they usually cut it in the proper length with a square pointed spade, which was about 24".
Look for "Trail Ace" about half-way down this document. There might be something you can use.http://www.fhwa.dot.gov/environment/fspubs/96232802/962802lo.pdf