Macro photograph of crystals in Herm Granodiorite with a xenolith

The delightful small island of Herm lies just a short boat ride away from Guernsey in the Channel islands. The entire island is made of the Herm Granodiorite (de Pommerai and Robinson 1994). This is an intrusive igneous rock that formed below the surface of the earth, probably during the later part of the Cadomian age which lasted from about 550 to about 700 million years ago. The southern part of the island is higher than the northern part. In the south, the rock comprises a plateau with a height around 60 metres. The rock has been extensively quarried and exported in the past. The stone is particularly hard and ideal for structures like kerbstones; examples of these still feature on the Thames Embankment in London.

To the north, the area is covered by wind blown sand that hides an old wave-cut platform, glimpses of which can be seen as jagged low-lying reefs offshore.. The sand is wholly composed of shells with not only fragments but also a high proportion of undamaged miniature molluscs and sea urchin tests. The underlying Herm Granodiorite is similar to some varieties of the Bordeaux Diorite occurring in Guernsey – typically made of feldspar and quartz with some biotite and hornblende crystals. One of the main characteristics of the Herm Granodiorite is the inclusion of many contrasting lumps of other igneous rock types known as xenoliths. There is a good exposure of this rock type on Mouisonniere Beach near a stone obelisk on the marram covered dunes. [The obelisk is a navigation marker that has been constructed on the site of an earlier Neolithic standing stone (dolmen) which was taken away by quarrymen in the 19th century].

The rocky outcrop on the sandy beach is full of xenoliths. It is of special interest to geologists because of the variety in the composition and shape of the xenoliths indicating a series of different processes were involved. Some are dark, angular and made of diorite. Others are paler and more rounded; often they have a rim of darker material. The combination of crystal types and sizes varies in each type of xenolith compared with the rock in which it is embedded. The causes and possible circumstances that led to the formation of these different sorts of xenoliths are the subject of much discussion among the experts.


British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, pp 56 – 62.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 1 – The St Peter Port Gabbro, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pp 4 -5.

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