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Saturday, 12 October 2013

Erratic cobbles and hammerstones

Modern hammerstones used in experimental archaeology in the United States.  Some of them have "battering marks" and fractures resulting from constant percussive contacts with the "object" stone -- and it is often stated that such stones become rounder and rounder with use, as they lose their irregularities.  So  after prolonged use become almost spherical........

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:

Typical fracture damage found on two glacial erratics on the flank of Carningli.  The larger boulder is sub-rounded, and after the main fracture damage was done, there was clearly further glacial transport which has smoothed off the rough edges.  The stone on the left has a rounded surface away from the camera, but it is all that is left of an earlier much larger boulder which has split apart on two fracture planes more or less at right angles.  This is quite severe damage......

Where we see abundant striations on cobbles or boulders, we can be pretty sure of glacial transport (see pics below), but not all rock types striate properly, so we have to look at other features instead.  Most geomorphologists are pretty good at identifying glacially transported stones, and at Rhosyfelin we see many of them embedded in the till and also found in secondary positions where they have been eroded out of the till and caught up in colluvial or slope deposits that have accumulated post-glacially.  Here are some of them:

Now there MIGHT be real hammer-stones and mauls at Rhosyfelin -- until one has examined the stones designated as such by the archaeologists it's difficult to say anything meaningful.  But I am sure the archaeologists are aware of the dangers of circular reasoning here. Because this site is by definition a quarry, all rounded "convenient" stones are therefore hammerstones or mauls.  Because we find nice convenient stones in the excavations, that proves that the site is a Neolithic quarry.......  Round and round in circles, and to hell with logic.

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.


geocur said...

It's worth mentioning the countless examples from experimental archaeology that we know were hammerstones but would never be recognised, because they finished up in pieces, or had no evidence of use wear .
I found a possible a few years ago that had been possibly used in secondary percussion to make some rock art ,scroll down to the 24th pic .

chris johnson said...

Very interesting.

I found this link

Faces in the rocks on Carn Ingli.

Amazing what you can see when you try.

BRIAN JOHN said...

Thanks Chris -- yes, you have to try REALLY hard with this one.....

Dave Maynard said...

This is easy, I can say with great authority, that they are all gastroliths - stones that dinosaurs carried round in their crops. Can anyone prove me wrong?

Just because the answer might be correct for one or two examples, doesn't make it applicable to all.

The same might be said to some of the comments here, with the greatest respect to everybody!


TonyH said...

Article on ECTON MINES: prehistoric to 19th Century Copper Mines in Staffordshire IN British Archaeology, Nov/Dec 2013 pp 18-23 includes on page 21, illustrations of hammerstones found in 2008-9; use damage is mostly at their ends; used for rock breaking and crushing ore in the Bronze Age

Anonymous said...

"This is easy, I can say with great authority, that they are all gastroliths - stones that dinosaurs carried round in their crops. Can anyone prove me wrong?"

Dave: There's enough confusion already without introducing the geological timeline from Christian Theology!

They'll be saying the stones were ballast from Noah's Ark next!

BRIAN JOHN said...

Robert has probably said that already......