ng, just as a steel knife
becomes thin and narrow by use. Scratches and scorings and polished
surfaces are found in all rocks that pass one another in close contact.
Its worn-out tools the glacier drops at the point where its ice melts.
This great, unsorted mass of rock meal and coarser debris the stream is
gradually scattering down the valley.

The name "moraine" has been given to the earth rubbish a glacier
collects and finally dumps. The _top moraine_ is at the surface of the
ice. The _lateral moraines_, one at each side, are the debris gathered
from the sides of the valley. The _ground moraine_ is what debris the
ice pushes and drags along on the bottom. The _terminal moraine_ is the
dumping-ground of this mass of material, where the ice river melts.

Glaciers, like other rivers, often have tributary streams. A _median
moraine_, seen as a dark streak running lengthwise on the surface of a
glacier, means that two branch glaciers have united to form this one. Go
back far enough and you will reach the place where the two streams come
together. The two lateral moraines that join form the middle line of
debris, the median moraine. Three ice-streams joined produce two top
moraines. They locate the lateral moraines of the middle glacier.

The surface of a glacier is often a mass of broken and rough ice,
forming a series of pits and pinnacles that make crossing impossible.
The sun melts the surface, forming pools and percolating streams of
water, that honeycomb the mass. Underneath, the ice is tunnelled, and a
rushing stream flows out under the end of the glacier. It is not clear,
but black with mud, called _boulder clay_, or _till_, made of ground
rock, and mixed with fragments of all shapes and sizes. This is the meal
from the glacier's mill, dumped where the water can sift it.

"Balanced rocks" are boulders, one upon another, that once lay on a
glacier, and were left in this strange, unstable position when the
supporting ice walls melted away from them. In Bronx Park in New York
the "rocking stone" always attracts attention. The glacier that lodged
it there, also rounded the granite dome in Central Park and scattered
the rock-strewn boulder clay on Long Island. Doubtless in an earlier day
the edges of this glacier were thrust out into the Atlantic, not far
from the Great South Bay, and icebergs broke off and floated away.

[Illustration: Potsdam sandstone showing ripple marks]

[Illustration: _By permission of the A