I've been doing some reading on hammerstones -- those nice handy cobbles which are just about right to hold in your hand when you are bashing flakes and irregularities off a piece of flint or some other stone which you want to make into an axe-head or an arrow-head, or a cutting implement, or whatever. People have been using hammerstones for millions of years, ever since humans started making implements and weapons, and wanted to shape large stones into smaller useful items.
In the wild, as it were, such stones are incredibly difficult to identify, and many rounded cobbles of a convenient size have of course been labelled as hammerstones simply because flint flakes have been found in the vicinity, giving rise to the idea of "flint knapping factories" or knapping floors. In some cases the percussive damage or battering marks are quite clear, and we might accept that stones with these marks on them are genuine signs of human involvement in stone shaping and tool manufacture. But in other cases great care is needed, since rounded or sub-rounded stones are commonplace in nature, in locations where abrasion of one stone against another is commonplace -- for example on storm beaches, in turbulent rivers and in fluvial and fluvioglacial sediments. We find many rounded and sub-rounded stones in glacial deposits too, and in such deposits there is an added complication in that signs of shearing and fracturing are much more common than in water-lain deposits. This is because cobbles in transport within and beneath glacier ice can be subjected to intense pressure, where one stone or cobble can literally be forced onto or into another -- if one stone is harder than the other, or if it simply rests in a more convenient position in the ice, we can get conchoidal fractures or facets created. Indeed, we sometimes find rough new facets and older smoothed facets, as illustrated on one of my older posts about a stone near our summer cottage in Sweden:
We can also see rough facets on this small boulder of Cambrian (?) purple sandstone, found on the flank of Carningli last year:
So until somebody shows me a hammerstone that is utterly convincing, I prefer to remain entirely sceptical, and to believe that the rounded and sub-rounded stones collected up from within the till layer and above it are entirely natural.
Finally, here are some nice photos of striated erratic cobbles and stones from around the world. No doubt about glacial action here. Note that some of them have rather good facets and fractures which could, in certain circles, be interpreted as "percussion fractures" caused in the tool-making process.