Braye Harbour from Rozelle Point at sunset
The Orbicular Diorite on Alderney was originally described by the eminent petrologist Stephen (Nocky) Nockolds, in the Geological Society's Magazine in 1931.
"The Orbicular Diorite was first found in a small embayment forming part of the west side of Corblet's Bay. It was found later in a less well developed fashion, at Saye Bay. It is quite possible that extended search would prove the presence of more. For the most part the orbicules are widely scattered, but a boulder was found at the Corblet's Bay locality which shows several associated with inclusions of a more basic rock. This boulder is now in the Sedgwick Museum, Cambridge."
These sites have subsequently been visited by geologists and students of petrology, but are in a fairly hazardous location, and the coordinates are also hard to obtain due to their rarity and potential value to collectors.
During World War 2, the island was occupied by the Germans, and became the most heavily fortified element of their Atlantic Wall. We surmise the outcrop of Orbicular Diorite featured in this EarthCache was exposed by cutting an access road to the Rozelle Battery, and was therefore not visible in 1931.
Orbicular Diorite
Orbicular Diorite on Alderney
Diorite is a plutonic rock with a granular texture that is sometimes alternatively known as black granite. Comprised chiefly of plagioclase feldspar and small amounts of hornblende, biotite, and other dark-colored minerals, the rock exhibits a characteristic appearance usually described as “salt and pepper.”
Though diorite may occur independently, it is often associated with granite and gabbro intrusions, with which it sometimes merges. The rock is generally produced in the continental crust above subduction zones and is found in some forms worldwide. However, orbicular diorite is an extremely rare variety of the rock that contains many spherical lumps, or orbs, that exhibit concentric zones of light and dark color. Once only known on the island of Corsica, occurrences of orbicular diorite have now been found in South Africa, North Carolina, and a few other locales as well (including Alderney). Yet, how this unusual type of diorite forms has yet to be determined with certainty and is a matter of significant debate among geologists.
Diorite varies somewhat in overall color depending on the percentage of dark minerals it contains, but is often dark grey to black. The rock, which also frequently exhibits a dull green tint, is sometimes utilized as a building material and as an ornamental stone. However, current demand for diorite is relatively low, perhaps because of its dark coloration. The rock seems to have been more popular in antiquity when many artists used the readily polishable material to create beautiful sculptures, many of which have survived to modern times and can be viewed at museums around the world.
An Orbicular Diorite and Ormalu Mantle Clock circa 1830 (Sothebys Auction Catalogue)
Geology of Alderney from Wikipedia
The geology of Alderney includes similarities in its rock to the neighbouring Normandy and Guernsey. Although Alderney is only five kilometers long, it has a geological history spanning half of the life of the earth. It is part of the Armorican Massif.
Relics of sediments appear as xenoliths in granites. However the earliest dated rock is the grey coloured Western Granodiorite from 2,220 million years ago in the Paleoproterozoic. As its name suggests it is found in the west end of Alderney. The xenoliths in it are dark ellipses that demonstrate that the rock has been squashed. This granite in turn was intruded by the Telegraph Bay Granite in the southernmost part of the island. This granite contains 50 mm feldspar crystals. Aplite veins continued from the same magma. The final stage of intrusion was a microgranite forming many dykes. Feldspar in the pink microgranite is only 2 mm big.
The next stage of geological history was the intrusion of the Central Diorite Complex that makes up the north and centre of the island. This belongs to the Cadomian Orogeny time at 600 to 500 million years ago. Embedded in the diorite are a couple of large gabbro inclusions, as well as a picrite on the east of Braye Bay. Some of the diorite has orbicular structure, concentric spheres of plagioclase and hornblende rich zones form balls up to 20 cm in diameter. A pale coloured granite intruded on the north: the Bibette Head Granite. This contains many xenoliths. Sodium rich dykes then were intruded.
In the next stage the terrane was uplifted, and eroded. Fine grained sand that formed quartzite was deposited. Further weathering ensued, with most of this deposit removed and laterite formed. Next a stream channel formed over the land, dumping coarse sand with feldspar. This formed a pink sandstone.The flow came from the northwest, with particles derived from granite and gneiss. initially this filled in the hollows in the underlying granites, but soon overflowed into a braided channel. Flood plain conditions caused layers of silt to form between the sand. These sediments deposited in the Cambrian are probably the final stage of the Cadomian Orogeny.
In the Variscan Orogeny folding and faulting affected all the rocks. Dolerite (or diabase) and lamprophyre dykes intruded. These are probably from the Carboniferous period.
In the Pleistocene varying sea levels caused raised beaches to form 8, 18 and 30 meters above the current sea level. As in Jersey, loess blew in as dust from the bare ground in the near glacial conditions in the ice ages. Head also formed in the periglacial circumstances by breaking off rock fragments and mixing with dirt (Regolith).
The Rozelle Point Outcrop and Your Task
Orbicular Diorite at Rozelle Point
An official tourist map can be obtained from the Alderney Visitor Information Centre in Victoria Street, St Anne, or other outlets around the island. The track from Whitegates to Fort Albert is usable by most island cars, and we recommend you park below Fort Albert on the tight right hand bend and admire the view. Walk down the track 100m and take the rougher track on your right , towards Rozelle Point. If your mobility is poor, this track can be taken in a car, with care. When the track turns sharply left, you have reached the outcrop on your left.
You will find examples of Orbicular Diorite scattered about across the rock face - some more visible than others, including the one in the image above.
Your tasks to claim this cache are:-
1. Measure a number of “orbs” and determine their average diameter.
2. From the information given, and your observation of the location, where else would you suggest examples could be found on Alderney, and why.
3. (Optionally) Post a photograph of the view with your log, but do not include these Diorites.
You may post your log, but it will be deleted unless you email your answers to the cache owner (juliadream) via our geocaching profile, or direct to Geocaching@JuliaDream.EU
We wish to acknowledge the help and support given to us by local residents, who brought this site to our attention.
Finally - strictly no “samples” to be taken without prior authority!