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 débris, 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.
Potsdam sandstone showing ripple marks
By permission of the American Museum of Natural History
Glacial striæ on Lower Helderberg limestone
Glacial grooves in the South Meadow, Central Park, New York
