Title strip


Augen within the solid rocks
At location 4 the augen gneiss (or orthogneiss) forms the solid rock. Although the rock appears to be a cold light grey, close up we can seen that it's a stripy light and darker grey with narrow bands of small black flecks.

Photo rock surface with veins
Large rock surfaces with augen smoothed by the grinding action of sediment in the river
This rock is a medium to coarse grained, banded rock. There are many large white-light grey inclusions surrounded by areas where the stripes are deformed in the shape of eyes (hence "augen" the German for eyes).
The rock composition
This rock has roughly the same minerals as that of granite, with quartz dominant (50%) (translucent crystals) and lower amounts of feldspar (35%) (whitish and less obviously crystalline) and lower still amounts of mica in the form of biotite (black-brown-metallic platy crystals) and muscovite (clear and lustrous often paper thin crystals) (10-20%) . It's this mix of minerals which creates its fairly neutral (if stripy) colour.
The metamorphism
However, like the rock at location 1, the rocks here have been metamorphosed. As we have moved further into the Gorge we can see the ravine sides towering above us and we realise that the gorge has cut down into rock by about 500m. These rocks are today at some distance below the "Real ground level" and at the time of metamorphic change some 280-300 million years ago were even more deeply buried.
Metamorphic grade
As the rocks at this location were buried deeper, they were subjected to greater pressure and, probably even more, to a higher temperature. This results in changes to the texture of the rock, and in the balance of minerals. These are progressive, and result in a series of "grades" of metamorphism.
photo of gneissic banding
The stripy banding is typical of medium to high grade metamorphism
Stripy Structures
The stripes in higher grade metamorphic rocks are an example of so called "compositional segregation" (see the close up photo above). Under the influence of the pressure and temperature of deep burial (several km underground), recrystallization occurred slowly with a realigning of the platy mica crystals in a direction at right angles to the direction of the pressure. The quartz (light in colour) and biotite (the black form of mica) separate out in bands of around 2-3 mm and 1-3 cm also at right angles to the line of pressure.
For more inf. on the origin of the augen and the rock texture, go to augen.


Photo of the locality
Down at the stream where the augen are first seen

The Augen (porphyroblasts)

These are typically lozenge shaped, up to 10 cm long but more typically 5 or 6 cm. If you get the sunlight in the right direction it's clear that the white central part of the auge is one large crystal as the whole of it reflects at the same angle. The crystal is of alkali feldspar (a crystalline mineral made of silica, whitish although there are inclusions of other minerals such as biotite.
Group of large augens
It is, maybe, the "augen" or yeux (eyes) which are the most striking feature of this rock.
The gneissic banding shapes
As can be seen in the photo above the augen appear to exert an influence on the shape of the stripes. These seem to be swirling around them. It's likely that the large crystals at the centre of the eyes were present at the time of deformation and metamorphism. As the rock was squashed, the banding was formed. As the space in the matrix diminished, the bands became more deformed round the large crystals. This all happened very slowly in spite of the fact that some of the shapes are reminiscent of liquid movements.
Quartz veins with folds
Other structures are the quartz veins (2-3 cm) like the one in the picture on the left. It is clear from this that the rock has deformed. The gneiss (mostly because of the platy crystals) is more ductile than the quartz. It compresses "gracefully", the parallel platy crystals sliding over each other whilst the quartz vein crumples to form z shaped folds. This is an example of a contrast in the competence of these two lithologies. The competence of a rock is a measure of its ability to resist stress without deformation.
photo of foldsin quartz veins
Z folds in resistant quartz veins in the more ductile gneiss

Navigation bar
Go to References