On the extreme west of the basin lie the Cotswold Hills of Gloucestershire. Here the Thames is born. The rain which falls on the hill-tops makes its way steadily into the soil, and is retained there. Down and down it sinks through the porous limestone and chalk, till eventually it reaches a layer of impenetrable material—clay, slate, or stone—through which it can no longer pursue its downward course. Its only way now is along the upper surface of the stratum of impermeable material. Thus it comes in time to the places on the hillsides where the stratum touches the open air (see diagram on p. 8), and there it gushes forth in the form of springs, which in turn become tiny streams, some falling westwards down the steep Severn valley, others running eastwards down the gentler declivity. At their northern end the Cotswolds sweep round to join Edge Hill; and then the hill-wall crosses the uplands of that rolling country which we call the Central Tableland, and so comes to the long stretch of the East Anglian Heights, passing almost continuously eastward through Hertfordshire and North Essex to Suffolk. On the south side the ring of hills sweeps round by way of the Marlborough Downs, and so comes to the long scarp of the “North” Downs, which make their way eastwards to the Kentish coast. Within the limits of this ring of hills the valley lies, not perfectly flat like an alluvial plain, but gently, very gently, undulating, seldom rising more than two or three hundred feet above sea-level, save where that great ridge of chalk—the Chiltern-Marlborough range—straddles right across the basin at Goring. Standing on one of the little eminences of the valley we can survey the scene before us: we can watch the River for many miles winding its way seawards, and note in all directions the same fertile, flourishing countryside, with its meadows where the soft-eyed cattle browse on the rich grass; its warm, brown plough-lands; its rich, golden fields of wheat, oats, and barley; its pretty orchards and farms close at hand; its nestling, tidy villages; its little pointed church steeples dotted everywhere. We can see in the distance, maybe, one or two compact little towns, for towns always spring up on wide, well-farmed plains, since the farmers must have proper markets to which to send their supplies of eggs, butter, cheese, and milk, and proper mills where their grain may be ground into flour. It is a pleasant, satisfying prospect—one which suggests industrious, thrifty farmers reaping the rich reward of their unsparing labours; and it is an interesting prospect, too, for this same prosperous countryside, very little altered during half-a-dozen centuries, has done much to establish and maintain the position of the Thames as _the_ great river of England.