We have a chalk field on a slope and in the last 5 or so years there has been increasing erosion and upheave. What can we do to stop it getting worse? There is a spring running underground nearby and it is grazed most of the year by sheep. It hasn't been reseeded for many years and thistles have started taking hold despite yearly topping around July. Any thoughts? Or any ideas where I can get good, unbiased proper advice? Many thanks.