A secret in Railway Land! And a prize to be won!

Can anyone tell me what creature made this and what for? I found two this afternoon in Railway Land Nature Reserve, both on the ground at the base of Hawthorn trees. I've never seen them before. They are about the size of a golf ball. Whoever gives me the correct answer first will win a signed copy of my book; * 'Secret Railway Land' * (Available to preview and or purchase, along with all my other books at www.blurb.com/bookstore)