Page:Popular Science Monthly Volume 4.djvu/665

Rh The percentage of stones present is variable. They are most common in the hilly districts, while in the lowland region the clay may predominate. Most of them show markings all over. They vary in size from grains to blocks several feet or even yards in diameter. Their shape is peculiar. They are neither round nor oval like the pebbles in river-gravel, or the shingle of the sea-shore; nor are they sharply angular like newly-fallen débris at the base of a cliff; but seem to be like the latter in general shape with the sharp corners and edges smoothed away. They are to the geologist what hieroglyphics are to the Egyptologist—the silent but impressive records of an age long passed away.

In narrow valleys the till often accumulates in such amount as to cover the solid floor many yards in depth. In such cases, the surface may be level, and, in the subsequent periods, the streams have made excavations in the mass, leaving the till in the terrace-form. Its unstratified character will be determined by examining the earth along



the sides of the escarpment, as, superficially, it is difficult to distinguish the material from the terraces of later age.

When the till is removed from the underlying rocks, their upper surface almost invariably shows a smoothed and often highly-polished appearance, and the whole pavement is marked with those peculiar scratches or striæ that form so characteristic a feature of the embedded stones. The extent to which the polishing is carried depends very much upon the nature of the rock. As the best-preserved stones of the bowlder-clay consist of close-grained limestone and clay iron-stone, so the same materials in the ledge-condition preserve most perfectly