Arthur led us to exposures on both sides of the road at the entrance to the car park. To the north the low cliff consists of siltstone units up to 1m in thickness interbedded with purplish mudstones showing thermal spotting. The rocks dip variably between 60 degrees to 40 degres to the North East and are well folded; these are the marine sediments of the Hornfelsed Brioverian shale formations, which show sole casts and ripple marks. The shales are between 700 to 900Ma and extend through the island from west coast to the south coast; they contain mudstone, siltstone greywacke and sandstone.
To the south of the road loose boulders on the beach showed concretions about 100mm in diameter. The wave-polished surfaces show minor faulting, flame and dewatering structures. Deformation across the beds of mudstone had bent the beds rather than fractured them indicating that they were plastic when the movement occurred. Immediately above this structure was an undisturbed bed, which would indicate a possible erosion surface, all dipping at 60° to 40°. The mudstone contained rounded blocks, which appeared to have fallen into the sediment indicating local land upheaval.
Moving northwards towards Le Pulec Bay the road cut through a gully in the shale formation, which had become filled with well-stratified head. Beds of larger boulders were clearly definable together with imbrication apparent on the smaller clasts; the head becomes more loessic upwards. It transpires that this rather fine road leading to the bay had been constructed solely for the extraction of seaweed to be used as an agricultural field dressing.
There was certainly no shortage of raw material, as we slid down the slipway at the head of the bay we were warned by Arthur to be rather careful where we walked as the seaweed was up to a metre deep in places.
The contact surface of the shales and granite could be clearly seen. There was considerable folding in the shale beds adjacent to the granite and aplite dykes which cut diagonally through the shales. The chilled margins were thin and there was little other visible evidence of thermal metamorphism. Northwest granite is the largest of the three principal masses on Jersey and has been dated at 480Ma.
Three mineral veins ran southwest along the length of the bay and disappeared out to sea. At least one of these veins had been worked commercially until the 1870's and deep seaweed filled excavations marked the extent of the workings, which could only be accessed at low water. Two of the lodes produced the minerals galena, sphalerite and ankerite whilst the third contains ankerite, tetrahedrite and silver sulphides with a little lead. The sphalerite (zinc blende ZnS) weathers to a dark brown colour whilst ankerite (Ca(Mg,Fe)(CO3)2 weathers to light brown and is the principal gangue mineral of the lode. All the minerals could be clearly identified at several locations along the major vein.
Back in the car park Arthur showed us a few specimens from his rock and mineral collection including suspicious looking yellow minerals gingerly removed from a lead lined box. Remind me not to put specimens in my trouser pocket when collecting in granitic areas!
There are four quarries to the north of the road from Le Pulec Bay; we visited one to look at another exposure of the granite shale junction. This disused quarry had been used for building quality granite extraction and traditional methods had been used. Blasting sets up micro-fractures which speed up the weathering process and render the granite unsuitable for construction purposes. The contact surface showed little sign of alteration other than a thin chilled margin with no apparent hardening of the shale. A thin vein of disseminated molybdenite ran across the granite down the contact surface and into the shales.
Lawrie Bubb.