Updated understanding of cave formation and habitation potential at 20+ caves at Sanna

The most energetic and talkative group of people to ever coinhabit Sanna came together last week to conduct a complete survey of every cave, crack and fissure at Sanna, Træna,

lead by expert speleologist Prof.emeritus. Stein-Erik Lauritzen (UiB).

The survey was initiated by Helgeland Museum, facilitating a workshop of multidisciplinary researchers all with interests in the geology, archaeology and history of Træna and the Helgeland region. This overlapped perfectly with the research objectives of the ARCAVE project and turned out to be overwhelmingly productive.

Previously, more than 20 rock shelters and caves were known from Sanna, systematically surveyed during Gutorm Gjessing´s original excavations at the island in the late 1930s alongside geologist Ole Grønlie. We revisited all known caves and rock shelter, while also scrambling up and down the rock face of most of the island, investigating every fissure we could find.

Fig: Noteworthy sites at Sanna island. Map by archaeologist Gutorm Gjessing (1943:8).

Compared to existing records, this resulted in a significant expansion of the number of rock shelters at the island - but most significantly, the keen eye and deep knowledge of Lauritzen did wonders to updating the geology of Sanna and the formation of the caves.

While initially thought to mostly being sea caves, formed by waves eroding the rock and thus typically producing pear-shaped cross sections, all except 1-2 appear to be the product of exfoliation and secondary frost erosion: cracks forming from the bedrock shedding off the outer layers as the previous surrounding land mass was carved away by multiple ice ages. When first formed, frost and thawing cycles have worked at the fissures and resulted in caves and rock shelters of various sizes.

The potential for unexplored habitation cave sites has been strengthened. Particularly considering that we frequently observed cave entrances blocked by massive rock fall deposits.

Thanks to Helgeland Museum and Marit Bertheussen for organizing this workshop and bringing us all together!

Fig: Båthellaren is the lowest elevation cave at Sanna and the only one displaying distinct sea cave morphometry.

Fig: Exfoliation caves. Rauhellaren rock shelter to the right and Trollhellaren to the left.

Fig: The dimensions of several of the cave entrances are quite spectacular. Mouth of Kleivhellaren.

Fig: Massive roof fall from Kleivhellaren.