The Scilly granite is very similar to that found in Cornwall, and was classified by Barrow (1906) into different types, the main ones being the older, coarse-grained G1, which is found mainly around outer the rim of the islands, and G2, which is finer-grained and is mostly in the central part of the islands and has often been intruded into G1. The two types of granite merge into each other without any obvious line of demarcation. Characteristically the granites are made up of quartz and crystals of feldspar, muscovite mica, biotite mica and other
FIG 7. Loaded Camel Rock at Porth Hellick, St Mary’s, May 2006. (Richard Green)
minerals. Much of the rock is beautifully striped through with veins and dykes, mostly narrow and usually white or black according to the infilling; white quartz or black tourmaline crystals can be found in these dykes, and rarely larger crystals of amethyst quartz. A vein of beautiful amethyst quartz that runs through some of the rocks to the north of St Agnes at one time had large, visible crystals, but most have since been taken by collectors. It is still possible to find smaller veins of crystals, and some beach pebbles have small layers of crystals running through them.
Another characteristic of the granite is the weathering along the veins and softer areas in the rock. The cooling and pressures have already formed these into very distinctive vertical and horizontal cracks, and erosion by weather and the sea then combine to produce the most extraordinary natural sculptures. Some of the most impressive examples can be seen on Peninnis Head and along the east coast of St Mary’s, but all the islands have examples. Some of these rocks have been given fanciful names: Pulpit Rock, Monk’s Cowl, Tooth Rock, Loaded Camel (Fig. 7). A curious type of formation seen frequently in Scilly is a rock basin in the top of a granite boulder where rainwater has weakened feldspar and released quartz crystals, which apparently have blown round and round to form a natural bowl.
Surface geology
When the granite weathers it eventually becomes reduced to sand. Blown sand is an important constituent of the soils of all the islands as well as forming the beautiful white beaches and the sand bars that link many of the higher parts of the islands. Ram (also known as head or rab) is a cement-like material formed by the breakdown of periglacially frost-shattered granite fragments that forms deposits around the bases of granite carns, in valleys and especially in the cliffs (Fig. 8). It is often excavated from ‘ram pits’ to be used by the islanders as a mortar in building work and sometimes as a road surface. Alluvium is found under the Porthloo fields, and at Lower and Higher Moors. Small patches of gravel found near the daymark on St Martin’s and at a few other places are probably of glacial origin – see below.
FIG 8. Ram shelf at the base of the cliff on Samson. (Rosemary Parslow)
FIG 9. An example of a raised beach at Porth Killier, St Agnes, where former beach levels can be seen above the present beach. May 2003. (Rosemary Parslow)
Raised beaches
Raised beaches are especially common throughout the southwest of Britain, and the Isles of Scilly have many examples. The raised beach at Watermill is a classic site with a conglomerate of clast-supported rounded cobbles and boulders, overlain by well-sorted medium sand (Selwood et al., 1998). There are many places all around the coasts in Scilly where former shore levels with beach deposits are exposed in the cliffs above the present beaches. There are raised beaches at Hell Bay, Bryher; Porth Killier, St Agnes (Fig. 9); Piper’s Hole, Tresco; Shipman Head, Bryher and many other places.
Glaciation
Although it was long thought that glaciation had missed Scilly, there is evidence that a tongue of ice from the southern edge of the Late Devensian ice sheet, the Irish Sea Ice Stream, probably reached the northern islands of Scilly 18,000 years BP (before present), eventually leaving deposits on White Island off St Martin’s, and on Northwethel (Scourse et al., 1990). The evidence for this lies in a great variety of rocks exotic to Scilly such as flint, sandstone and associated ‘erratics’. The best example of glacial till in the islands is within the Bread and Cheese
FIG 10. The bar to White Island is a former glacial feature, probably a glacial moraine. June 2002. (Rosemary Parslow)
formation at Bread and Cheese Cove SSSI on the north coast of St Martin’s: the overlying gravels, the Tregarthen and the Hell Bay gravels, are interpreted as glaciofluvial and solifluction deposits respectively. There are erratic assemblages with both deposits (Selwood et al., 1998). Recent work suggests that some of these deposits, such as that at Bread and Cheese Cove, may not be in their original positions (Hiemstra et al., 2005). Other sites with glacial links occur in the bars in the north of the islands, such as the ones at Pernagie, the one connecting White Island to St Martin’s (Fig. 10), and Golden Bar, St Helen’s: these are probably glacial moraines, not marine features (Scourse, 2005).
EARLY HISTORY – THE SUBMERGENCE
Twenty thousand years ago most of Britain was under the last glaciation, extending as far south as the Wash and south Wales. At this time sea level would have been as much as 120m below Ordnance Datum. Then the climate ameliorated and by 13,000 BP the Devensian ice had almost disappeared (Selwood et al., 1998).
Four thousand years ago, before the sea inundated the land, Scilly would have had a very different landscape, with low hills and sand dunes surrounding a shallow plain (Fig. 11). Based on the present-day undersea contour lines, there would at that time have been three main islands: the principal one would have included the present-day St Mary’s, Tresco, Bryher and St Martin’s, the Norrard Rocks, the Eastern Isles and the St Helen’s group; Annet and St Agnes would have made up a smaller second island group; and the Western Rocks would have been the third. Later the islands became parted as the sea rose still further. The long isolation of St Agnes from St Mary’s and the rest of Scilly may possibly explain the differences in the flora – for example why the least adder’s-tongue fern Ophioglossum lusitanicum is restricted to St Agnes.
Most accounts of the submergence of the Isles of Scilly are based on the model proposed by Thomas in Exploration of a Drowned Landscape (1985). Thomas suggests that sea level rose rapidly and reached to within a few metres of present-day levels by 6000 BP, although final submergence of the island of Scilly to
FIG 11. A map showing how the main islands may have appeared prior to the submergence. (After Thomas, 1985)
create the present archipelago may not have been effected until post-Roman times. Archaeological and historical evidence show that although sea was rising on a unitary island about 2000 BC, ‘submergence began in earnest during Norman times and was effectively completed by the early Tudor period’ (Thomas, 1985; Selwood et al., 1998). However, Thomas recognised that although his model assumes a gradual process of submergence, there is an alternative picture with a series of dramatic events such as tidal surges. According to Ratcliffe and Straker (1997), submergence may have been even more gradual than Thomas proposes. The most controversial aspect of Thomas’s model is his suggestion that separation of the islands did not occur until