If it was a swamp and is now dry it seems as if the ground was drained at some time. If it still wet and unworkable in the winter a French drain will help. Dig a trench deeper than your usual planting depth and fill it with all your stones or any rubble you can get. But if it does stay dry, loads and loads of manure over the next few years to lighten the ground up. I had the same problem on a plot I used to have, I ended up using raised beds which I could have had no dig, but I used to turn them over in the traditional style.