LVI.— The Hereford Earthquake of December 17, 1896, considered in relation to Geological Structure in the Bangor-Anglesey Region .

Abstract

CONTENTS . 1. PHYSIOGRAPHY. 2. ROCKS COMPOSING THE DISTRICT. 3. STRUCTURAL RELATIONS. 4. SEISMIC EFFECTS OBSERVED. 5. CONCLUDING CONSIDERATIONS. 1. Physiography .—Broadly speaking, Carnarvonshire and Anglesey consist of portions of two ancient and deeply denuded plateaux. The higher of these, which may be called the mountain plateau, is part of that very extensive massif out of which has been sculptured the mountain country of North Wales as a whole. It ends off somewhat abruptly in Carnarvonshire along a NE.-SW. line drawn from near Conway towards Nevin, and passing a little to the NW. of Llanberis. Seen from Anglesey its profile appears as that of a very flattened dome, some forty miles in length, and reaching a maximum elevation of 3500 feet. The lower plateau has an average height of about 300 feet, and comprises the island of Anglesey and a part of the mainland some four miles wide, lying to the NW. of the line above mentioned, and may be called the Bangor-Anglesey or the Menai plateau. The Menai Strait is merely the deepest of a series of well-marked NE.-SW. valleys by which it is traversed. 2. Rocks composing the District .—The mountain plateau is composed of Cambrian and Ordovician rocks (the latter largely volcanic), all of which are more or less cleaved and hardened by compression, forming a thick and very resistant mass. The Bangor-Anglesey plateau, with which we are more immediately concerned, is composed of rocks which for seismological purposes may be grouped as follows:— Glacial Drifts. Carboniferous rocks. Ordovician series.