The rhyolite quarries at Bømlo in Norway: Traces of firesetting in the Neolithic

A shallow hole in the rock. Traces of firesetting at the Siggjo Neolithic rhyolitie quarries. Photo by Per Storemyr

A shallow hole in the rock. Traces of firesetting at the Siggjo Neolithic rhyolite quarries. Photo by Per Storemyr

A couple of days ago I climbed the mountain Siggjo in Western Norway together with my family. Siggjo is renowned for its deposits of the volcanic stone rhyolite that was heavily used for arrow heads and other tools from c. 4000 to 2500 BC in the Norwegian Neolithic. Finally I got to see the great traces of firesetting that are present in these hilltop quarries! The traces were interpreted by archaeologist Sigmund Alsaker almost 30 years ago, and we relied on them as an important reference when conducting experiments with firesetting in the North-Norwegian Mesolithic Melsvik chert quarries two years ago – experiments that I have previously reported in my blog.

Siggjo is the highest mountain at the island Bømlo south of Bergen in Western Norway. It is an important regional landmark, especially for sailors, at a distance looking like a volcano. Just north of the peak at almost 500 metres above sea level, a vein of very fine grained, chert-like rhyolite crops out in a little valley. There is nothing to be seen for the untrained eye (except beautiful scenery!), but on closer inspection dozens of convex forms and shallow depressions in the bedrock becomes apparent.

These forms may have a diameter of more than one metre, and they are typically connected to spalling of the bedrock: multiple “flakes” have loosened from the rock perpendicular to the surface. On comparing with natural weathering forms, this type of spalling makes no sense, and it is obvious that other “rock-breaking” mechanisms must have been at work.

Convex traces of firesetting in the Siggjo Neolithic rhyolite quarries. Photo by Per Storemyr

Now, Sigmund Alsaker, back in the 1980s, found lots of charcoal, dated to the Neolithic (by C14), in waste heaps close to the convex rock forms. Thus, there can hardly be another interpretation of these forms than that they originate from the use of fire back in the Neolithic. Alsaker also found similar forms in the famous greenstone quarries at Hespriholmen, off the Bømlo coast, during his work in the 1980s. Together with Stakaldeneset further north (by Florø), the islet Hespriholmen is the most important production site for stone axes in the Norwegian Stone Age (Mesolithic to Neolithic). It is comparable to the “Axe Factories” at Great Langdale in the UK Lake District, where firesetting was also at work in the Neolithic.

Since firesetting seemingly was the most important quarrying method for the rhyolite at Siggjo, as well as for the greenstone at Hespriholmen, Alsaker carried out experiments. He was able to break loose greenstone by the use of a rather big fire, and so it is more than likely that his hypothesis was correct.

This is the rhyolite at Siggjo: fine-grained and banded. Photo by Per Storemyr

However, there is more to breaking loose rock by fire than by big fires only. Firesetting is a method with multiple facets. You can make big fires with high temperatures and spall off rock in a rather uncontrollable manner – or you can set small fires one by one with lower temperatures, taking advantage of the stone properties, listening to how the rock slowly cracks and creaks as stress is built up. Thus, you may be able to break loose stone for cores that are only little affected by temperature alterations.

It was along the latter line of experimentation that we carried out firesetting at the Melsvik chert quarries two years ago. We were able to produce convex forms of spalling in the rock that were very similar to the forms that were produced back in the Mesolithic.
Importantly, these forms are also very similar to what we observed in the Siggjo rhyolite quarry: Loads of shallow, convex, semi-circular forms with spalled rock visible along the periphery. Thus, it is entirely possible that the people that exploited the hilltop quarry at Siggjo back in the Neolithic knew how to take advantage of small, targeted fires that broke loose rock just the way they wanted.

Here are some impressions from Siggjo (scroll down to references and map)