A closed Bouguer gravity low is revealed by land and marine gravity survey data centred on Holyhead Bay, northwest Anglesey. The anomaly extends across areas on land and offshore mainly occupied by Precambrian rocks. A concealed granite offers the best explanation for the anomaly and a late Caledonian age seems most probable for the intrusion, although Precambrian–early Cambrian ages are also possible. The existence of a granite in this area could have influenced the development of mineralization and the low density rock mass also formed a relatively buoyant core to the basement block. It could therefore have been instrumental in the preservation of this Precambrian structural high.