Problem soil: sandy topsoil on clay sub soil
I have just started digging some new beds from lawn. The topsoil (a little more than a spades depth) is very sandy but the subsoil contains large lumps of sticky red clay. I'm planning on planting things that are suitable for a seaside garden, and was going to dig in some organic matter to the topsoil to help the plants along, but should I try to improve the clay layer, or leave well alone? Any help appreciated!