[Goosegirl]

No need to remove the lighter clay. You (by digging and adding stuff) then worms, frost, potatoes and leeks will all help break up the sub-soil clods over time. My sub-soil was tractor-compacted silt with only about 6" max top soil, but I dug the beds by digging a trench and waggling a fork in the clay at the bottom to loosen the "pan" then forked in manure, etc. Over the last few years, there seems to be less and less clay showing up when I dig as I have added more compost, manure (from our sheep), lime and gritty top soil to top up the raised beds as the soil level fell, and bashed any clods up to mix them with the good stuff.