Clay & stones.
Clay usually has all the nutrients needed for plant growth it is the fine particles of it that cause the prob stopping drainage and closing down the air paths.
clay can easily be made into friable soil by adding rotted material that makes Humus , add horticultural grade sharp sand ( stuff that is not sea side sand with shells aka lime in it ) , add 4 to 6 mm grit .
The first couple of years are the worse but if you double dig ( with a mini excavator Hee hee ) down to say a metre and layer the infill including soil in 100 mm layers things soon shape up .
Sprinkling in lime helps crumb the clay so does bags of opened plaster that would normally be thrown away & are instead liberally sprinkled on /into the soil . .
I made my first five level beds on hard dried out blue clay and 2 inch stones in what used to be an ancient pond bed that used to be in the middle of farm pastures where the new housing estate was built in 1982 .
The mini digger was a real boon . the top layer got a decent dose of frost
After the first year from construction I re dug the beds out by hand and heaped it all on the path ways , made sure it was fairly well remixed and added a big barrow of black rotted horse turds & one level barrow of sharp sand per metre run ..each bed was 36 inches wide by 30 feet long with 24 inch concreted pathways all round . There was a massive amount of worms right through the new soil .
The next crops were fantastic .. I'll see if I still have a piccky of them & if so will photo bucket them into the post .