Large blocks of stone, quite different from the rocks on which they lie, are very numerous, and are called "erratics," since they are evidently wanderers from a distance. Sometimes such blocks can be proved to have been brought many miles from their home among the higher peaks. The long lines of stones and mud seen on the sides of a glacier are called "moraines," and at the end of every glacier we find a big heap known as a "terminal moraine." But the stones of which they are composed are probably not to be entirely accounted for in this way. Can we not conceive that the weight and pressure of a descending glacier may be sufficient to break off many protruding portions of the rocky bed over which it flows, and then to drag them along with it? This seems reasonable. Let us therefore consider the materials of which moraines are composed to be derived partly from the rocks beneath and partly from those above the glacier. But whatever their origin, such materials must inevitably find their way to the end of the glacier and be added to the big heap there. The work of transportation is then taken up by the stream which always flows from the end of a glacier. Such streams are in summer-time laden with fine sediment, which gives them a milky and turbid appearance. Thus a glacier wears away the rocks over which it flows; rock fragments become embedded in the ice, and these are the tools with which a glacier does its work. It must be granted that the downward movement of a great mass of ice is irresistible, and consequently that as the moving glacier slowly creeps along, it must inevitably cause the stones which it thus holds to grind over the surface of the rock. It is easy to imagine the effects of this grinding action. If sand-paper, rubbed for a minute or two over wood, wears down and smooths its surface, what must be the result of all these stones, together with sand and mud, grinding over the rocky bed?