The _fissures_, which, when of large size, are called _crevasses_, are formed in those portions of a glacier where, from some cause, the ice is subjected to slight tension. This occurs especially where, through irregularities in the bottom, the slope of the descent is increased. The ice, then, instead of moving in a continuous stream at the top, cracks open along the line of tension, and wedge-shaped fissures are formed extending from the top down to a greater or less distance, according to the degree of tension. Usually, however, the ice remains continuous in the lower strata, and when the slope is diminished the pressure reunites the faces of the fissure, and the surface becomes again comparatively smooth. Where there are extensive areas of tension, the surface of the ice sometimes becomes exceedingly broken, presenting a tangled mass of towers, domes, and pinnacles of ice called _seracs_. [Illustration: Fig. 6.--Section across Glacial Valley, showing old Lateral Moraines.] Like running water, moving ice is a powerful agent in _transporting_ rocks and earthy _débris_ of all grades of fineness; but, owing to the different consistencies of ice and water, there are great differences in the mode and result of transportation by them. While water can hold in suspension only the very finest material, ice can bear upon its surface rocks of the greatest magnitude, and can roll or shove along under it boulders and pebbles which would be Unaffected except by torrential currents of water. We find, therefore, a great amount of earthy material of all sizes upon the top of a glacier, which has reached it very much as _débris_ reaches the bed of a river, namely, by falling down upon it from overhanging cliffs, or by land-slides of greater or less extent. Such material coming into a river would either disappear beneath its surface, or would form a line of _débris_ along the banks; in both cases awaiting the gradual erosion and transportation which running water is able to effect. But, in case of a glacier, the material rests upon the surface of the ice, and at once begins to partake of its motion, while successive accessions of material keep up the supply at any one point, so as to form a train of boulders and other _débris_, extending below the point as far as the glacial motion continues. Such a line of _débris_ is called a _moraine_. When it forms along the edge of the ice, it is called a _lateral_ moraine. It is easy to see that, where glaciers come out from two valleys which are tributary to a larger valley, their inner sides must coalesce below the separating promontory, and the two lateral moraines will become united and will move onward in the middle of the surface of the glacier. Such lines of _débris_ are called _medial_ moraines. These are characteristic of all extensive glaciers formed by the union of tributaries. There is no limit to the number of medial moraines, except in the number of tributaries.