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.

The boulder clay at the seaward end of the Vale seems to have been capped by ice of a thickness of nearly 100 feet which efficiently contained the waters of the lake until they overflowed through a depression among the hills to the south of Malton.  If the waters escaped by any other outlet to the west near Gilling and Coxwold, it can scarcely have been more than a temporary affair compared to the overflow that produced the gorge at Kirkham Abbey, as the Gilling Gap was itself closed by the great glacier descending the Vale of York.  The overflow of the lake by this route, south of Malton, must have worn a channel down to a lower level than 130 feet O.D. before the ice retreated from the seaward end of the Vale, otherwise the escape would have taken place over the low hills blocking the valley in that direction and the normal course of the drainage of the country would have been resumed.  The southern overflow evidently dug its way through the hills fast enough to maintain that outlet, and at the present time the narrow gorge at Kirkham Abbey is only 50 feet above sea level, and the hills through which the Derwent passes at this point are from 200 to 225 feet high.

[Illustration:  A Map of North-Eastern Yorkshire showing Lake Pickering during the maximum extension of the ice.  The area covered by ice is left unshaded.  The arrows show the direction of the glacier movements.  (Reproduced from the Quarterly Journal of the Geological Society, by permission of Professor Percy F. Kendall.)]

As the waters of the lake gradually drained away, the Vale was left in a marshy state until the rivers gradually formed channels for themselves.  In recent times drainage canals have been cut and the streams embanked, so that there is little to remind one of the existence of the lake save for the hamlet still known as The Marishes.  The name is quite obviously a corruption of marshes, for this form is still in use in these parts, but it is interesting