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Geology and weathering

The type of rock that the Tanum rock carvings are carved into is called Bohus granite and can be found from Gullmarsfjorden in the south to Østfold in the north. The Bohus granite was formed 920 million years ago, 15 kilometres below the surface at that time. The surface on which the images are made was smoothed by the latest ice age, which started 115,000 years ago and lasted until 11,600 years ago. After the ice age, Bohuslän was underwater, but slowly the crust pressed down by the ice began to rebound. The rock surfaces on which the carvings are made started to emerge above sea level about 5,000 years ago.

Granite weathers away

The rock carvings are highly sensitive to weathering because they are often shallow, and it doesn’t take much erosion for the images to fade away. Weathering starts when the rock comes into contact with rainwater or condensation, which dissolves the least resistant minerals. This leads to larger and larger areas being affected, and over time, the strength of the rock diminishes. The weakened rock can then be attacked by more aggressive weathering, such as frost action, which can break off a significant amount of rock in a short period of time.

Weathering can also have a protective effect – minerals and oxides released during weathering can accumulate on the surface and prevent the rock from absorbing water. Unfortunately, strong chemicals have been used to clean the carvings in the past 100 years. This has removed the protective layers, and the granite has started to absorb water again, allowing weathering to continue with renewed force. Apart from external influences, such as frost action and careless cleaning, the quality of the granite plays a significant role in how quickly and extensively it weathers. Some rock carvings have completely disappeared, while others remain almost intact.

Previously, there was a belief that acid rain was the main cause of weathering. This has been proven to be an incorrect conclusion, likely inspired by the real problem of buildings and artwork made of other types of rocks weathering away in city centres across Europe. Granite itself is acidic and therefore withstands acid deposition well, unlike, for example, limestone and marble.

Weathering of the rock surface at Aspeberget. Photo: Eva Ernfridsson.

What can we do?

The most important thing we can do to protect the rock carvings is to keep them dry and ensure they have a consistent temperature. Water can be diverted using sandbags placed above the panels, and severely damaged carvings can be covered with waterproof and insulating materials that allow for the release of condensation. Covering also prevents the growth of lichens and algae on the rock, which can contribute to weathering. Ethanol is used to clean the granite, as it kills vegetation and prevents new growth while not affecting the protective surface of the rock.

An important task in preserving the rock carvings is to photograph them and document their appearance. This work is also crucial for advancing research on the rock carvings. The images are made accessible to archaeologists and other interested individuals through publications and the internet.

Cleaning of the Vitlycke panel. Photo: Hans Lundenmark.