For many years it was postulated that an ice sheet had covered the north and west of Great Britain during the period when Palæolithic man lived and made his flint implements in the south and east of England. It was taught that the southern margin of this ice sheet corresponded in the main with an imaginary line stretching from about the mouth of the Severn to the Wash, or, as others claimed, from the mouth of the Severn to the mouth of the Thames. These views were held to be supported by the fact that very few palæoliths had been found north of the above-mentioned line. As years passed, however, new facts testifying to the lengthy duration of the Palæolithic period slowly accumulated, and knowledge respecting the great Ice Age gradually increased. True, many and divergent views have been advanced respecting the several phases of the Ice Age as these concern Lancashire and the north of England generally. Some glacialists hold that the Ice Age there, as also in the Alps, was broken up into several complete cycles consisting of a number of glaciations with a corresponding number of interglacial periods. Others believe that there was only one interglacial period in the north of England.