=Subsidence of great deltas.= As a rule great deltas are slowly sinking. In some instances upbuilding by river deposits has gone on as rapidly as the region has subsided. The entire thickness of the Ganges delta, for example, so far as it has been sounded, consists of deposits laid in open air. In other cases interbedded limestones and other sedimentary rocks containing marine fossils prove that at times subsidence has gained on the upbuilding and the delta has been covered with the sea. It is by gradual depression that delta deposits attain enormous thickness, and, being lowered beneath the level of the sea, are safely preserved from erosion until a movement of the earth's crust in the opposite direction lifts them to form part of the land. We shall read later in the hard rocks of our continent the records of such ancient deltas, and we shall not be surprised to find them as thick as are those now building at the mouths of great rivers. =Lake deltas.= Deltas are also formed where streams lose their velocity on entering the still waters of lakes. The shore lines of extinct lakes, such as Lake Agassiz and Lakes Bonneville and Lahontan, may be traced by the heavy deposits at the mouths of their tributary streams. * * * * * We have seen that the work of streams is to drain the lands of the water poured upon them by the rainfall, to wear them down, and to carry their waste away to the sea, there to be rebuilt by other agents into sedimentary rocks. The ancient strata of which the continents are largely made are composed chiefly of material thus worn from still more ancient lands--lands with their hills and valleys like those of to-day--and carried by their rivers to the ocean. In all geological times, as at the present, the work of streams has been to destroy the lands, and in so doing to furnish to the ocean the materials from which the lands of future ages were to be made. Before we consider how the waste of the land brought in by streams is rebuilt upon the ocean floor, we must proceed to study the work of two agents, glacier ice and the wind, which coöperate with rivers in the denudation of the land. [Illustration: Fig. 87. Section of Undifferentiated Drift near Chicago] CHAPTER V THE WORK OF GLACIERS =The drift.= The surface of northeastern North America, as far south as the Ohio and Missouri rivers, is generally covered by the drift,--a formation which is quite unlike any which we have so far studied. A section of it, such as that illustrated in Figure 87, shows that for the most part it is unstratified, consisting of clay, sand, pebbles, and even large bowlders, all mingled pell-mell together. The agent which laid the drift is one which can carry a load of material of all sizes, from the largest bowlder to the finest clay, and deposit it without sorting. [Illustration: Fig. 88. Characteristic Pebbles from the Drift