Certainly feels like it has been shaped to fit tightly into the right-hand.

Indeed, the rock does not have any signs of impact points (Knapping) which would be clearly visible had it been manufactured.  It looks like it has been subjected to water erosion and it is coincidental that it resembles an axe head.

It does not resemble any of the identified forms of handaxe nor would it be suitable for a later hafted axe. 

See below for Acheulean and Mousterian handaxes:

I agree with Mark Keiter diagnosis

I think you would have to hold it in your right hand to know it fits like a glove. Although there is flint in the area I believe this to be of igneous origin.

There are many naturally occurring stones which are mistaken for manufactured items.  The most fascinating is the Makapansgat pebble.  This artifact was found near remins of Australopithecus Africanus and was transported to its site of discovery perhaps 3 million years ago.  It is considered one of the earliest pieces of hominid art.  The features are naturally occuring but it is believed that the early hominids recognised the features of a face and took it with them.   

I think you only have to hold it to know. I also discovered it close to many Portal Dolmens and Archaeology going back to both the Neolithic and Mesolithic periods. Don't think it has to be flint to be an artifact.

If you look at the pictures below you will see that a number of the naturally formed pebbles have the rough shape of a handaxe.  They are however formed by erosion in rivers.  

The bluestobe pebbles in particular are prone to form these shapes, which are quite common.  Handaxes are manufactured by a highly skilled craft of knapping which produces working surfaces with sharp edges.  They are finely balanced for complex tasks.  Returning to the stone you have found you need to ask what it would have been used for.  It clearly has no cutting faces and if had ever been used by meso or neolithic people as a hammer stone it would have surface damage visible on the striking face.

There is little doubt that it is a naturally occuring pebble.

Hi Barry,

Thanks for the answer. There seems to be lots of definitions of Knapping - in terms of the stone types "Knapped". I'll try and dig out some more examples of Stone Axes found here - which have definitely been worked - although I wouldn't call it "Knapping".