STRUCTURAL ARCUATION IN THE BERWYN HILLS, NORTH WALES

New mapping has better defined the well known arc in Acadian (late Cal edonian) structures centred in the Berwyn Hills. This arc, convex to t he north-west and about 20 km across, is defined by the strikes of bed ding and cleavage and the trends of fold hinges. In contrast, a minera l lineation within the cleavage has a north-north-westerly plunge thro ughout the arc. It approximates to the long (X) axes of measured strai n ellipsoids. A primary origin for the arcuation is implied by this co nsistency, by the absence of secondary cleavages and by opposing sense s of fold transection across the arc. The new data favour the hypothes is of shortening of the Lower Palaeozoic cover against and over a pre- existing basement high inferred from geophysical data and from metamor phic and sedimentary facies.