La Corbière, St Aubins and Belcrute Bay

Sunday Afternoon 23 April

After explaining the archaeology of the sand dunes at St.Ouen's Bay (menhirs and ossuaries), Deirdre Schute led us on a flying trip to Corbière Point, to examine a particular granite block. This block illustrated three phases in the emplacement of the granite in SW Jersey at around 565 - 15 Ma during the final post tectonic alkali phase of the Cadomian igneous activity in Jersey (Adams 1976). At base, the hornblende-biotite granite was coarsely grained pale pink, above the base it was a medium grained deep pink. The whole block was intruded, appropriately for Easter, by a "hot cross bun" pattern of micro granite (aplite?), which was an even deeper pink in colour. The medium grained granite had darker greyish coloured flow banding patterns. Work that out if you can!

To remind us of our sea crossing to come, a sculpture of two hands clasping each other commemorated the French catamaran "St. Malo" striking the Frouquie rock 900m to the north of La Corbière lighthouse. Due to the swiftness of the rescue services no lives were lost as a result of this incident. Very reassuring.

St Aubin's was next, and we congregated at the slipway by the Royal Channel Islands Yacht Club HQ (how posh), where we met our next leaders Tony Ellison and Sandra Mahé. The c.700 Ma Jersey "shales" (Brioverian siltstones and mudstones at this locality) were seen to be dipping some 70º to the SE. An eroded dyke intruding the shales was faulted, with sinistral movement. We then walked south towards Belcroute Bay and saw a mica lamprophyre dyke exhibiting step faulting in a dextral sense. Analysis of this dyke has shown it to be potassium rich, with potassium feldspars, phlogopite, pyroxenes, amphiboles and mica. Despite the presence of mica (a weak, readily eroded mineral) in this dyke, it stood proud of the harder Jersey shale. This was attributed to the shales being severely fractured, and having a higher lime content than is usual, which makes them susceptible to dissolution in water.

Further south we crossed a bridge over a larger dyke of pink flow-banded rhyolite trending E-W. This is considered to originate from a parasitic vent of a nearby volcano. The Jersey shales in the vicinity were a lighter green in colour, showing that they had been baked by the rhyolite. Dolerite dykes were crossed then and, after crossing cobbled beach deposits, the marginal facies of the SW Jersey granite were seen.

During this walk, the cliff deposits and structures were examined. What was seen are probably the finest sequences of Pleistocene periglacial head and raised beach deposits to be found anywhere in the British Isles. The sequences varied; this is shown in the sketches. Raised beach deposits had very well rounded granite cobbles with the occasional flints; head deposits had angular Brioverian fragments up to 2cm in size in a medium sand matrix. There are raised beaches at 8m, 18m and 30m above sea-level in Jersey and we saw clear evidence for the 8m one at Belcroute in the wave cut platform, notch and rounded pebbles. We estimated that the cliffs were between 20 and 25m high. Silty loess deposits, noticeably thickening to the south, lay between the head and the present day soils. The loess was remarkable for the sheer quantity deposited. It was very fine grained and silky-smooth (like yellow eye-shadow!).

Belcroute Bay marks the boundary (unexposed) between the shales and the granite.

The deduced geological history of the area is as follows:

1) (~ 700 Ma.) Pre-existing rock eroded to make submarine delta fan

2) Fan deposits lithified into the Jersey Shale formation

3) (~ 600 Ma.) Shale compressed and folded during Cadomian Orogony

4) Dykes and granite intruded into shale

5) More dykes emplaced through granite and shale

6) Everything uplifted

7) Glaciation produced head

8) Wind-blown loess deposited

(with thanks to the Société Jersiaise and apologies if we have misrepresented what they told us!)

Nigel Spencer & Lynn Everson