Significance:
The Northmaven intrusive complex occupies an area of 129 square km. In
the North Roe area it is composed mainly of the Ronas Hill
granophyre along with smaller units of diorites and gabbros which
are believed to be enclaves in the granophyre. A long and staged
history is envisaged for the emplacement of the complex.
Essentially the basic diorites and gabbros were emplaced first as
sheets followed by a two stage emplacement of granite and
granophyre, firstly as sheets then finally as a stock or plug. The
emplacement of dyke swarms soon followed this.
The granophyre is deep red in colour and has a drusy texture
with large feldspars enclosing crystals of milky quartz. A variety
of dykes with a North-South trend cut both the intrusions and the
country rock, and range in composition from basic dolerite to
quartz-porphyry and riebeckite-aegirine.
Shetland knives. Image courtesy of Noel Fojut
Beorgs of Uyea
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Felsite Dykes and Neolithic Axe Factory (HU 327 901)
The suite of dykes that include the axe factory are of green/blue felsite, some of which are spectacularly flow banded. Flow banding is due to the parallel arrangement of tabular crystals by the movement of magma into a fissure, in this case joints within the cooling granophyre.
The main ‘axe factory’ site is a trench, approximately 1m deep, which has been dug down against the west side of a felsite outcrop to form one wall of a gallery. The other side and end walls are of a rough dry-stone construction and were overlain by a roof of lintels composed of granite and felsite, some of which are still in situ.
This gallery was discovered in 1942 and was excavated in 1949. The felsite wall was found to have been quarried and undercut and the gallery floor buried in waste material from the manufacture of stone tools along with some of the workmen’s tools. It was also noted then that the surrounding hillside was covered in more broken rock than could be expected from natural erosion. More recently other, less elaborate, working platforms have been identified on the surrounding hillside, these are associated with felsite outcrops and waste material from the roughing-out of tools as well as semi-finished roughed-out tools.
It is believed that the roughed-out tools were removed from site for finishing. The end result of this Neolithic industrial enterprise was the production of highly polished stone knives, axes, adzes and maces; beautiful examples of these are on display in the Shetland Museum in Lerwick. Because of the lack of wear on these highly decorative examples it is supposed that they were manufactured for ceremonial rather than every-day use.
This spherulic and banded felsite is unique to this part of Shetland making the source of these tools readily identifiable.