An Acoustic Survey of the Sea Floor South of Dorset and Its Geological Interpretation

An area of sea floor from the Dorset coast southward to about 50° 30' N, and between 2° 2' and 2° 27' W has been surveyed in detail using asdic equipment for recording relief, and a gravity corer and free-swimming divers for obtaining samples. Neighbouring areas were examined in less detail. The asdic provides an acoustic picture of the sea floor, and enables outcrops and faults to be mapped. Supplementary information was obtained from Admiralty surveys. The submerged part of the Isle of Purbeck Anticline is shown to be an elongated dome with Corallian rocks exposed in its core at Lulworth Bank. The dome is intersected by many sub- parallel faults trending about 15° east of north. Except along the steep monoclinal northern limb dips are low, seldom more than 3° or 4°. Minor folds are superimposed on the main structure. In the east they lie nearly east and west, while in the west, where the Purbeck Anticline yields place to the Weymouth Anticline, a north-westerly trend is dominant. South of the Purbeck Anticline lies a broad and gentle syncline in Jurassic and Lower Cretaceous rocks which is named the Shambles Syncline. It is overlain unconformably by Upper Cretaceous rocks.