Spur Point geologically unique

With the ‘virtual’ States due to debate the proposed extension to the Longue Hougue reclamation area imminent, I felt compelled to express my concerns. As proposed, the scheme will effectively obliterate one of the most interesting and easily accessible geological sites in the island. This is in addition to the biological and environmental concerns expressed by others. The well-known adage ‘who pays the piper calls the tune’ appears to apply equally well to government departments who commission expensive ‘independent’ reports in order to justify a preferred course of action. The Environmental Impact Assessment report on the proposed Longue Hougue project compiled by Royal Haskoning falls very firmly into this category. Haskoning have form in this role (think anti-tank wall), but in the case of the Longue Hougue project their failure to perform due diligence regarding the geological significance of the site and their downplaying of its importance offends me greatly. It does them no credit whatsoever; their suggested methods of ameliorating the impact of the project are risible in the extreme – but more of this later, after a bit of history and an explanation of why the Spur Bay exposures are important.

In the early 1980s, a group of respected academic geologists, many with long-standing research interests in Guernsey, were commissioned to compile a modern geological map of the island. This was published by the British Geological Survey as one of their standard British Isles maps, in conjunction with the States of Guernsey, who had sponsored its production. Normally such maps also have an accompanying descriptive memoir, but as the funding for that was not forthcoming from the survey, the map’s authors sought an alternative publishing option locally. They found this in the shape of a newly established monograph series published by Guernsey Museum. The Outline and Guide to the Geology of Guernsey was published in 1991 as Museum Monograph No. 3. This academically rigorous and comprehensive work immediately became the go-to source of reference on Guernsey’s hard-rock geology. Its detailed descriptions of the rock types and their relationships are complemented by an extensive bibliography and a series of field itineraries, one of which takes in the outcrop of the St Peter Port gabbro from Salerie Corner round to Spur Bay. The rock features described at the Salerie have already been lost to the North Beach marina project and now it looks as if the Spur Bay section is also under threat.

So, what about the geology exposed there; why is it special? The St Peter Port gabbro was singled out for particular mention at a lecture delivered to La Societe Guernesiaise by Dr Chris Topley in 1986. Dr Topley was one of the authors of the aforementioned Outline and Guide… and an account of his lecture was published in the Societe Transactions for 1986 by Mick De Pomerai, co-authored by Dr Topley. Curiously, this article does not appear in the list of references given in the Longue Hougue impact assessment produced by Royal Haskoning. The following quotation is certainly an inconvenience to any arguments in favour of the extension:

‘Whilst this part of our coast may not be the most beautiful... its importance to the scientific community is immense; unfortunately the land reclamation scheme at Bulwer Avenue and the Marina Development in Town have already covered useful exposures and Dr Topley urged us most strongly to preserve those which remain, particularly between Richmond Corner and the Bulwer Avenue reclamation.’

Gabbro is a so-called ‘basic’ igneous rock which has cooled slowly from the molten state, deep in the Earth’s crust. The slow cooling allowed large crystals of the constituent minerals to grow, and their black and dark green colour will be familiar to all who walk past stone walls in St Peter Port.

Like many of the world’s gabbro intrusions, the St Peter Port rock shows a layered structure but, unlike most of the others, ‘our’ gabbro has an unusual mineral composition (with hornblende substituting for the more common pyroxenes) and the layering is present at several different scales. The hornblende within the St Peter Port gabbro occurs in several different forms. One form with platy crystals intergrown with paler feldspars is what rock quarrymen used to call ‘birds’ eye’, while layers with long prismatic hornblende crystals were called ‘long-grain’. More properly, this latter variety is known as bojite. The layered effect in the gabbro is produced by concentrations of the darker hornblende crystals alternating with layers containing paler minerals, principally feldspars.

The variable layering and gabbro types across the St Peter Port gabbro intrusion are of course visible all across the Belgrave outcrop at low tide, but the intertidal rocks are rugged and unforgiving, heavily obscured by marine algae. The Spur Bay exposures show most of these variations in a beautifully presented and easily accessible way.

In my time as Guernsey Museum’s natural history officer (now retired) and former Geology Section secretary of La Societe, I have conducted many geological site visits to Spur Bay, both for local and visiting geological groups. If it was located in another jurisdiction with a more enlightened planning framework, it would certainly be designated a Site of Special Scientific Interest and there would be no possibility of development without far more effective amelioration methods than those suggested by Haskoning. Simply taking a number of boulders off the beach and setting them aside doesn’t really fit the bill. Geologists need access to the rocks in situ in order to properly appreciate and measure their field relationships.

While it might seem that I only have negative things to say, I do have a more positive suggestion which could even create a tourist attraction. If the reclamation must go ahead and the landward margin of the reclamation was realigned parallel with, and perhaps just below, normal high water mark, the strip of shingle and beach head could be preserved as a recreational area and the important exposures would remain to be seen. I realise this would slightly reduce the available waste disposal area and increase the set-up costs, but it would leave access to a site of special interest which could be interpreted for the benefit and interest of all.

ALAN HOWELL MSC FMA

Clifton Villa,

Little St John Street,

St Peter Port,

GY1 2PR.

alan@cwgsy.net