What on earth ...?

This afternoon I was rooting around in an ice-age kame, a mound of sand, gravel and till left behind by a retreating glacier. I was hoping for some ancient shells but found none. What I did find was this object, the likes of which I have never before seen. It is about 10 cm in length and consists of a core of highly compacted, coarse greenish-yellow sediment encased in a hard "skin" which is flaking off. 
I have no idea what it is, but I hope that there are geologists out there in blip-land who might be able to enlighten me.