Nothing authentic is known of the history of Guernsey previously to its annexation to the Duchy of Normandy in the tenth century. The only sources of information as to events which may have occurred before that date are references in monkish chronicles of the usual semi-mythical type, and indications conveyed by cromlechs and menhirs, fragments of Celtic instruments and pottery, and a few Roman relics. It is unfortunate that we are thus precluded from acquiring any knowledge of the development of a people as to whom the soundest among conflicting conjectures seems to be that, coming originally from Brittany, they preserved the purity of the Celtic race through periods when in other offshoots of the same stock its characteristics were being obliterated by the processes of crossing and absorption.
If early local records had existed they would hardly have failed to have given minute details of the convulsion of nature which resulted in the destruction by the sea of the forest lands on the northern and western sides of the island, and in the separation of tracts of considerable magnitude from the mainland. Geologists are agreed in assigning to this event the date of March, 709, when great inundations occurred in the Bay of Avranches on the French coast; they are not equally unanimous as to the cause, but science now rejects the theory of a raising of the sea-level and that of a general subsidence of the island. The most reasonable explanation appears to be that the overpowering force of a tidal wave suddenly swept away barriers whose resistance had been for ages surely though imperceptibly diminishing, and that the districts thus left unprotected proved to be below the sea-level--owing, as regards the forests, to gradual subsidence easily explicable in the case of undrained, swampy soil; and, as regards the rocks, to the fact that the newly exposed surface consisted of accumulations of already disintegrated deposits.
It is unquestionable that before the inroad of the sea the inlet in the south-west of the island known as Rocquaine Bay was enclosed by two arms, the northern of which terminated in the point of Lihou; on which still stand the ruins of an old priory, while the southern ended in the Hanois rocks, on which a lighthouse has been erected. Lihou is at present an island, accessible only at low water by a narrow causeway; the Hanois is entirely cut off from the shore, but it is a noteworthy fact that the signs of old cart-ruts are visible at spring tides, and that an iron hook was recently discovered attached to a submerged rock which had apparently served as a gatepost; besides these proofs of the existence of roads now lying under the waves, it is said that an old order for the repair of Hanois roads is still extant. That Vazon and the Braye du Valle were the sites of forests is indisputable, though the former is now a sandy bay into which the Atlantic flows without hindrance, and the latter, reclaimed within the present century by an enterprising governor, formed for centuries a channel of the sea by which the Clos du Valle, on which the Vale Church stands, was separated from the mainland. A stratum of peat extends over the whole arm of the Braye, while as regards Vazon there is the remarkable evidence of an occurrence which took place in December, 1847. A strong westerly gale, blowing into the bay concurrently with a low spring tide, broke up the bed of peat and wood underlying the sand and gravel, and lifted it up like an ice-floe; it was then carried landwards by the force of the waves. The inhabitants flocked to the spot, and the phenomenon was carefully inspected by scientific observers. Trunks of full-sized trees were seen, accompanied by meadow plants and roots of rushes and weeds, surrounded by those of grasses and mosses; the perfect state of the trees showed that they had been long buried under the sand. Some of the trees and boughs were at first mistaken for wreckage, but the fishermen soon discovered their error and loaded their carts with the treasure locally known as "gorban." Subsequent researches have shown that acorns and hazel-nuts, teeth of horses and hogs, also pottery and instruments of the same character as those found in the cromlechs, exist among the Vazon peat deposits. There is therefore abundant evidence that the legends relating to the former inhabitants of the forest are based on traditions resting on an historical foundation.