That grey gault clay is the worst soil in creation. I was taught to plough on that and it was not a good place to learn.Someone probably used it without having the proper shares which should be 44K's, they stand up a bit higher at the back end as they use a thicker spacer at the front of the board. I have a brand new Kent board in the shed if anyone wants it, good boards on heavy clay which we have a lot of here in Kent