Otta

Rock type: Phyllite slate.
Age: Ordovician (approx. 460 million years).

Otta slate represents what was originally a clay/siltstone formation which during the Caledonian period was deformed and metamorphosed with the foundation of regular cleavage strata.

Good slate deposits occur in tight fold structures. Sheet thickness varies between 8 and 80mm.

In the lower part of a slate formation the phyllite contains large crystals of needle-like hornblend (Pillarguri and Høgsæter).

Pillarguri is exclusively quarried to the south of Ottadal, whilst Høgsæter is quarried on the north side. In certain areas the slate can be found with a rusty surface.

This rust colour occurs in cleavage planes which have been exposed to erosion from water and to oxygen.