The color of unweathered till depends on that of the materials of which it is composed.  Where red sandstones have contributed largely to its making, as over the Triassic sandstones of the eastern states and the Algonkian sandstones about Lake Superior, the drift is reddish.  When derived in part from coaly shales, as over many outcrops of the Pennsylvanian, it may when moist be almost black.  Fresh till is normally a dull gray or bluish, so largely is it made up of the grindings of unoxidized rocks of these common colors.

Except where composed chiefly of sand or coarser stuff, unweathered till is often exceedingly dense.  Can you suggest by what means it has been thus compacted?  Did the ice fields of the Glacial epoch bear heavy surface moraines like the medial and lateral moraines of valley glaciers?  Where was the greater part of the load of these ice fields carried, judging from what you know of the glaciers of Greenland?

Bowlders of the drift.  The pebbles and bowlders of the drift are in part stream gravels, bowlders of weathering, and other coarse rock waste picked up from the surface of the country by the advancing ice, and in part are fragments plucked from ledges of sound rock after the mantle of waste had been removed.  Many of the stones of the till are dressed as only glacier ice can do; their sharp edges have been blunted and their sides faceted and scored.

We may easily find all stages of this process represented among the pebbles of the till.  Some are little worn, even on their edges; some are planed and scored on one side only; while some in their long journey have been ground down to many facets and have lost much of their original bulk.  Evidently the ice played fast and loose with a stone carried in its basal layers, now holding it fast and rubbing it against the rock beneath, now loosening its grasp and allowing the stone to turn.

Bowlders of the drift are sometimes found on higher ground than their parent ledges.  Thus bowlders have been left on the sides of Mount Katahdin, Maine, which were plucked from limestone ledges twelve miles distant and three thousand feet lower than their resting place.  In other cases stones have been carried over mountain ranges, as in Vermont, where pebbles of Burlington red sandstone were dragged over the Green Mountains, three thousand feet in height, and left in the Connecticut valley sixty miles away.  No other geological agent than glacier ice could do this work.

The bowlders of the drift are often large.  Bowlders ten and twenty feet in diameter are not uncommon, and some are known whose diameter exceeds fifty feet.  As a rule the average size of bowlders decreases with increasing distance from their sources.  Why?

Till plains.  The surface of the drift, where left in its initial state, also displays clear proof of its glacial origin.  Over large areas it is spread in level plains of till, perhaps bowlder-dotted, similar to the plains of stony clay left in Spitzbergen by the recent retreat of some of the glaciers of that island.  In places the unstratified drift is heaped in hills of various kinds, which we will now describe.