No one looking at the present maps of the north of England and Scotland would be led to suspect what a large number of lakes must have been scattered over the surface of these northern regions when the Romans set foot in the country. Yet if he turns to old maps, such as those of Timothy Pont, published some three hundred years ago, he will notice many sheets of water represented there which are now much reduced in size or entirely replaced by cultivated fields. If, farther, he scans the topographical names of the different counties, he will be able to detect the sites of other and sometimes still older lakes; while, if he sets to work upon the geological evidence by actual examination of the ground itself, he will be astonished to find how abundant at comparatively recent times were the tarns and lakes of which little or no human record may have survived, and often how much larger were once the areas of lakes that still exist. Owing to some peculiar geological operations that characterised the passage of the Ice-Age in the northern hemisphere, the land from which the snow-fields and glaciers retreated was left abundantly dotted over with lakes. The diminution and disappearance of these sheets of water are mainly traceable to the inevitable process of obliteration which sooner or later befalls all lakes great and small. Detritus is swept into them by rain and wind from the surrounding slopes and shores. Every brook that enters them is engaged in filling them up. The marsh-loving vegetation which grows along their shallow margins likewise aids in diminishing them. Man, too, lends his help in the same task. In early times he built his pile-dwellings in the lakes, and for many generations continued to cast his refuse into their waters. In later days he has taken the more rapid and effectual methods of drainage, and has turned the desiccated bottoms into arable land.