Enthusiasts of ancient rock art, markings on stone made by prehistoric people, are developing a database of material found in Wales dating back millennia. The origin of the highly decoratively carved Llwydiarth Esgob Stone, Anglesey, for example, is unknown but it is likely to have been part of a burial-ritual monument.

The Welsh Rock Art Organisation is a non-profit research body specially interested in researching and promoting prehistoric rock art in Wales. Members record as much as they can about an artwork's size, shape, complexity and location and cross-reference their findings with those across Europe.

Some of their subjects remain stunning works of beauty many centuries after they were carved by their creators. The spirals, zig-zags and other art at the Stone Age chambered tomb Barclodiad y Gawres, Anglesey, are a relatively recent discovery.

Not all the rock art they seek out can be reached easily. And photographing it in situ can require lighting. Note the measuring stick to give the image scale.

Ancient but sophisticated farming societies worked hard at their art. The Llanbedr spiral stone, a clockwise 10-ringed spiral, was pecked away by hand. This pagan symbol is found on a stone first recorded near Dyffryn Ardudwy, Gwynedd, in the 19th Century. It now stands behind the font of a church.

One art form which the group are keen to record are cupmarks. These were a common way of creating art in prehistoric Europe and the patterns of their use can be related across many sites. Maen Cattwg, Caerphilly, has 30 clear cupmarks with another 18 that are too weathered to be photographed in daylight.

Photographing in the dark is sometimes the best way to highlight how much art is on an ancient stone monument. Up to 110 cupmarks are found on the capstone of Bachwen, a burial-ritual monument in Gwynedd. The jury is still out on whether all these are original or some are later Bronze Age "graffiti".

This small stone turned up in an excavation of a Medieval field boundary near Wrexham. The Caer Alyn stone is unusual because both cupmarks have been pecked rather than gouged. Probably Bronze Age, if not earlier, it is thought this is a fragment of a much larger stone or boulder with more cupmarks.

Not all the art is an ancient as it could be, however. Ty Illtud, a prehistoric chamber and capstone also has least 60 carvings, mostly lozenges, diamonds and crosses, that may well have been put there by generations of medieval shepherd boys.