After meeting up, just behind the Waxham great barn, on a crisp sunny winter's day, we walked down to the beach to look at the rocks associated with the walls and the off shore man-made reefs of the coastal defence system. Wendy Hamilton, the leader, gave a handout describing the construction of the reefs and effects. For more information about their construction see North Norfok Reefs

Unfortunately although the sea was a mill pond it was high tide so the exploration was limited to the rocks that made up the groynes and wall defence. After a short introduction explaining the rocks are from the same quarry in Norway, Wendy, posed the question what is the rock type? Hammering is not allowed but loose specimens can be collected but no damage is permitted to any part of the defence system. Off you go and find out!

At the first distant glance they're very dark grey/black = mafic = igneous! But one rock was light grey. The rocks shimmered due to the high percentage of crystal faces reflecting the sunlight. Closer inspection showed that the majority of the rocks are medium to coarse grained =1 cm and a dark-grey/blue/black colour with little finer grained matrix material present. Many crystals were foliated which gave a false impression of a metamorphic sedimentary rock and these were yellowish at the edges of the sheets. The standard characteristics of granite (pink and quartz) were not present (at the first site). Some rocks were almost pegmatic in crystal size.

There was no direction of the larger grains and they are subhedral to anhedral and granular and simple twinning was present in some crystal faces. The hardness test was limited (coin did not scratch but knife did) so was around 6 on Moh's scale. No fossils were present. Lens inspection gave very little quartz but large amounts of mica foliated type crystals (dark hornblende and biotite?).

The single lighter rock was very pale grey and black lath or lens shaped crystals (biotite?) were obvious and these crystals were very noticeable in the quarry drilled holes. This rock was finer grained and a bit more granite-like in appearance. It was equally granular with no noticeable direction with the black laths.

Several rocks displayed a polished face which gave the distinct impression of being machined. Other rocks had deposits (whether quartz or calcite could not be accurately determined as no-one brought any acid!) but this could indicate a deposition in a fault plane (figure 2). Although this was an igneous rock no inclusions of country rock were present. Of great interest were very small deposits of a bright copper/orange mineral looking like a pyrite present in a depression in one rock.

After a short inspection of the rocks and a review of the findings Wendy cleared up the mystery. The rocks are from the Larvik plutonic quarry in Norway and are an intermediate rock called Larvikite. This is an igneous rock noted for its lack of quartz.

Walking along the beach other rocks in defence wall were examined and some interesting examples were found.

One rock (figure 3) clearly showed the relatively sharp boundary between the quartz poor, darker, smaller grained Larvikite and the quartz rich, light pink, larger grained granite. This might be the incorporation of the local or crustal rock into the melt or a change in the melt composition but clearly the larger crystals require longer cooling which a chilled margin might provide.

Larvikite Details
This plutonic complex is some 1000 km² in area and is situated SW of Oslo reaching the coast. There are a series of semi-circular plutons emplaced into the local Cambrian, Ordovician and Silurian sediments during a 5-6 million year period between 297 Ma and 292 Ma (Permian) with the youngest pluton to the west.

Larvikite has a range of chemical compositions depending upon where in the plutonic complex the rock is found. Broadly speaking it is a variety of syenite or monzonite and it characterized by very low percentages of quartz with varying degrees of alkali feldspar and plagioclase (see QAPF diagram figure 5). It is an alkaline igneous rock often referred to as silicon under-saturated.

Augite, hornblende, amphibole, biotite, magnetite, pyroxene and others minerals may also present.

One feature that can be present is perthite (from the exosolution and lamination of the feldspar) and this gives rise to a 'Schillerizing or bronzing' effect which could explain the very small deposits of the pyrite copper/orange mineral but more importantly the characteristic blue shimmer.

The formation of Larvikite is a little confused. Looking at Bowens reaction series, pyroxenes, amphibole, biotite along with the Ca-Na plagioclase series all crystallize early from the melt with quartz and the alkali feldspars tend to be later. The melt producing the Larvikite being silicon under-saturated doesn't produce the quartz. However, some special events must have occurred with the melt to produce the pink quartz rich sharp boundary granites and the more diffuse amphibole crystal rich vein. Papers at the AGM please!

Whilst the rocks at Waxham are not of high quality one aspect of Larvikite is the overall blue /grey colour and in some cases vivid blue due to the anorthoclase (an alkali feldspar) which makes Larvikite sought after as a polished building facing stone. Royal blue pearl granite, blue Norwegian moonstone are some of more lurid names given to it.

After examining these rocks for some time the party drove to Horsey Gap to view the baby seals and I thanked Wendy for showing us some 'foreign aliens', unusual and interesting rocks in a mundane setting which the majority of people walking on the beach did not notice. I left the party thinking what a nice day to avoid the Christmas hassle but then Wendy's warnings came loud and clear 'Don't touch the seals they bite'.