Pebble spotting on Auchmithie beach, E Scotland

Where is Auchmithie beach ?

Outcrop of conglomerate overlain by sandstone on Auchmithie beach, near the harbour.

A short walk from Auchmithie village which is a few miles north of Arbroath in Angus region, E Scotland. There is a car park just behind the village hall.

What is a pebble ?

A rock sample that can fit in the palm of your hand [1]. Larger fragments are often called cobbles and boulders.

Why pebble spotting ?

A fun activity that I was involved in during Scottish Geology Festival, run by the Scottish Geology Trust in Late September. Plus this beach has many different types of pebbles that can be associated with the surrounding conglomerates which are clearly the main source for the pebbles. These conglomerates and sandstones are of Lower Devonian age.

The rock types ?

Quartzite, a metamorphosed sandstone, seems to be the most common pebble in the conglomerates and loose on the beach. On fresh surfaces, it has a sugary texture and is a much tougher rock than sandstones which are also found on the beach and in the conglomerates. The quartzite comes in various colours and there is a banded variety.

Regionally metamorphosed rocks such as schist and gneiss are common. Gneiss has a banded appearance with dark bands containing dark minerals like pyroxene and biotite and the lighter bands are made up of quartz and feldspar. You can find quartz and feldspar in the dark regions of a gneiss as is the case with the sample on the green mat below.

Some labelling of rock types for the pebbles within the conglomerate

There are also granites (red and white-grey varieties) and finer grained micro-granites. Sandstone pebbles are often quite flat as have been eroded along their bedding planes. The more unusual shape of the sandstone on the green mat below is due to it once being a piece of matrix material between the conglomerates. You can see a small piece (grey colour) of one of the pebbles attached bottom left of the sandstone sample.

Pebbles with a spotted appearance are common - these are volcanic rock, in this case a basalt with minerals that have filled the holes in the lava. This is called an amygdaloidal basalt.

One of the most sought after pebbles in this beach and other beaches south of the highland boundary fault (NE Scotland) is a silica variety called jasper. It is often used for jewellery such as necklaces. The most common colour of jasper is red and different shades of red are possible due to the various iron minerals it contains. It is possible to find yellow jaspers here and I came across green jasper further north at Boddin Point.

Deformation of the pebbles

Some of the pebbles in the conglomerates contain fractures. There is a clear fracture zone below and one of the pebbles (in particular) has been clearly displaced by one of those fractures. Some of these fractures have been filled by a white-pinkish coloured barite cement, although the timing of these is uncertain. Many of the fractures in the pebbles line up with the same trend meaning they were fractured after they were incorporated into the conglomerate. Other fractures have found a route around the pebbles. More work is needed to understand the order of events here.

Pebbles from Auchmithie beach

Fracture zone in the conglomerates at Auchmithie. A close up (right) shows how some of the pebbles have been sheared (arrow)

1.         Mitchell, C. The Pebble Spotter’s Guide (National Trust).