The superficial alterations in the appearance of these parts of Yorkshire were brought about by the huge glaciers which, at that time, choked up most of the valleys and spread themselves over the watersheds of the land. In the warmer seasons of the year, when the Arctic cold relaxed to some extent, fierce torrents would rush down every available depression, sweeping along great quantities of detritus and boulders sawn off and carried sometimes for great distances by the slow-moving glaciers. The grinding, tearing and cannonading of these streams cut out courses for themselves wherever they went. In some cases the stream would occupy an existing hollow or old water-course, deepening and widening it, but in many instances where the ice blocked a valley the water would form lakes along the edge of the glacier, and overflowing across a succession of hill shoulders, would cut deep notches on the rocky slopes. Owing to the careful work of Mr C.E. Fox-Strangways and of Professor Percy F. Kendall, we are able to tell, almost down to details, what took place in the Vale of Pickering and on the adjacent hills during this period. In the map reproduced here we can see the limits of the ice during the period of its greatest extension. The great ice-sheet of the North Sea had jammed itself along the Yorkshire coast, covering the lower hills with glaciers, thus preventing the natural drainage of the ice-free country inland. The Derwent carrying off the water from some of these hills found its outlet gradually blocked by the advancing lobe of a glacier, and the water having accumulated into a lake (named after Hackness in the map), overflowed along the edge of the ice into the broad alluvial plain now called the Vale of Pickering. Up to a considerable height, probably about 200 feet, the drainage of the Derwent and the other streams flowing into the Vale was imprisoned, and thus Pickering Lake was formed.