The continental glaciers manage in many cases to convey detritus from a great distance. Thus, when the ice sheet advanced southwardly from the regions north of the Great Lakes, they conveyed quantities of the _debris_ from that section as far south as the Ohio River. In part this rubbish was dragged forward by the ice as the sheet advanced; in part it was urged onward by the streams of liquid water formed by the ordinary process of ice melting. Such subglacial rivers appear to have been formed along the margins of all the great glaciers. We can sometimes trace their course by the excavation which they have made, but more commonly by the long ridges of stratified sand and gravel which were packed into the caverns excavated by these subglacial rivers, which are known to glacialists as _eskers_, or as serpent kames. In many cases we can trace where these streams flowed up stream in the old river valleys until they discharged over their head waters. Thus in the valley of the Genesee, which now flows from Pennsylvania, where it heads against the tributaries of the Ohio and Susquehanna, to Lake Ontario, there was during the Glacial epoch a considerable river which discharged its waters into those of the Ohio and the Susquehanna over the falls at the head of its course. [Illustration: _Front of Muir Glacier, showing ice entering the sea; also small icebergs._] The effect of widespread glacial action on a country such as North America appears to have been, in the first place, to disturb the attitude of the land by bearing down portions of its surface, a process which led to the uprising of other parts which lay beyond the realm of the ice. Within the field of glaciation, so far as the ice rested bodily on the surface, the rocks were rapidly worn away. A great deal of the _debris_ was ground to fine powder, and went far with the waters of the under-running streams. A large part was entangled in the ice, and moved forward toward the front of the glacier, where it was either dropped at the margin or, during the recession of the glacier, was laid upon the surface as the ice melted away. The result of this erosion and transportation has been to change the conditions of the surface both as regards soil and drainage. As the reader has doubtless perceived, ordinary soil is, outside of the river valleys, derived from the rock beneath where it lies. In glaciated districts the material is commonly brought from a considerable distance, often from miles away. These ice-made soils are rarely very fertile, but they commonly have a great endurance for tillage, and this for the reason that the earth is refreshed by the decay of the pebbles which they contain. Moreover, while the tillable earth of other regions usually has a limited depth, verging downward into the semisoil or subsoil which represent the little changed bed rocks, glacial deposits can generally be ploughed as deeply as may prove desirable.