Bowlders, Or Boulders, loose rounded blocks of stone, named by the French blocs erratiques, found scattered over the surface in high northern and southern latitudes, extending to within 35° or thereabouts of the equator. In the northern hemisphere they are always of the varieties of rock which are found in solid ledges in a northerly direction; and in the southern hemisphere the ledges are again met with toward the pole. These loose rocks appear in each case to have been transported toward the equator, either by glaciers or by icebergs, and to have been subjected to rolling action, which has rounded off their corners and ground their surfaces. (See Diluvium.) The size of these transported blocks is often enormous. At Fall River, Mass., on the S. side of the bay at the mouth of Taunton river, a bowlder of conglomerate rock was uncovered in the gravel resting on granite ledges which was estimated to weigh 5,400 tons. The ledges of this conglomerate are met with only on the other side of the bay. Along the coast of New England the bowlders constitute by their great numbers and size a marked feature in the landscape. They are sometimes found perched upon bare ledges of rock, and so nicely balanced that, though of great weight, they may be rocked by the hand.
These are called rocking stones. " Plymouth Rock" is a bowlder of sienitic granite, ledges of which are found in the towns near Boston. The highest mountains are often covered with these bowlders of the drift formation. Upon the bare granite summit of Mt. Katahdin - the highest mountain in Maine - at an elevation of 3,000 feet or more above the surrounding valleys, pieces of limestone containing fossil shells are found, though no ledges resembling them are known except many miles to the northwest, and at a much lower level. The northern and central parts of Europe are equally interesting for the distribution of bowlders. The pedestal of the statue of Peter the Great at St. Petersburg was hewn out of a granite bowlder, weighing about 1,500 tons, that lay on a marshy plain near the city. Upon the limestone ledges of the Jura mountains are found bowlders of granite which must have come from the higher Alps, where ledges of similar character are found. Some of these bowlders are of very large dimensions, one in particular, known as the pierre d Martin, according to Mr. Greenough, measuring no less than 10,296 cubic feet, and weighing consequently about 820 tons.