Though Louis Agassiz developed his glacial theory from studies in Switzerland, he possessed an imagination that ran before the knowledge of his time. Swiss glaciers are now so limited that they are of very little use to us when we seek to explain the origin of boulder-clay. In arctic and antarctic lands, however, we meet with continental glaciers, many miles in width, moving across lowlands, in virtue of the pressure from some great snow-dome, to which additions are continually being made behind them. Even when fed by diminished snow-fields, like those in Spitsbergen, these glaciers dominate the landscape and form the principal rock-masses over hundreds of square miles. Such glaciers gather into their lower portions all the loosened material on the hill-slopes and valley-floors. With the tools thus supplied, further material is plucked from jointed or fissile rocks as the mass moves forward. Freezing and thawing at the base of the great ice-sheet, as water flows here and there beneath it, further disintegrate the rocky floor. The broad ice-sheet sinks in a mass of broken rock and sludge at one point, and at another drags this mixed material forward as an abrading agent. The lower half of such a glacier, or the whole thickness of it near its front, where surface-melting has removed the higher layers, is in reality an agglomerate of stones and mud held together by an ice-cement (Fig. 12). When an epoch of advance is over, when the ice-sheet stagnates and its frozen constituent melts away, it becomes more and more like a boulder-clay as time goes on. True boulder-clay then forms its surface, while ice remains plentiful below. [Illustration: Fig. 13. Arctic Glacier and Boulder-Clay. The Sefström Glacier, Ekman Bay, Spitsbergen, in 1910, with boulder-clay in foreground, marked by kettle-holes, and deposited by an advance of the glacier over Cora Island in 1896.] Since the stony matter is not evenly distributed, some parts of the surface sink more quickly than others, through loss of a greater portion of their former bulk. Roughly circular pits or "kettle-holes" appear, in which water gathers. The water running from these washes across a part of the boulder-clay, bears off the mud, and leaves bands of sand and gravel. The clayey portion thus removed may accumulate as a fine deposit in other outlying pools, and is interstratified, when the flow of water is temporarily increased, with coarser and more sandy layers. Ultimately, the frozen water of the groundwork drains away, and only the stones and clay of the ice-sheet remain upon the field. They form, however, a very important residue, weathering in steep cliffs and pinnacles in the dry air of the arctic lands. The boulder-clay thus left shows a sharply marked boundary where the edge of the stagnating ice-sheet lay. It is, in fact, the surviving part of the complex sheet, and now undergoes moulding, like other rocks, by atmospheric agencies (Fig. 13).