The structure of a delta, shown in Fig. 189, shows its history. At any stage in its growth the river discharges its sediment across that part of the platform already built. The sediment rolled at the bottom of the current is dumped on reaching the steep slope, and constitutes the inclined _fore-set_ beds shown in Fig. 189. The material in suspension is carried farther, settles more gradually, and constitutes the _bottom-set_ beds (_c_, Fig. 189). In time the bottom-set beds, originally deposited some distance beyond the debouchure, may come to be overlain by the fore-set beds, deposited at a later time. While the fore-set beds are being deposited on the steep slopes of the delta, and the bottom-set beds beyond, deposition is also taking place on the top of the delta. These _top-set_ beds are laid down in a nearly horizontal position, and their seaward margin is gradually extended. Thus the delta comes to have the threefold structure shown in Fig. 189. That part of the delta which is above the abrupt slope of its front corresponds in all essentials to an alluvial fan; but the delta as a whole differs from the fan in its abrupt and crenate or digitate margin. It is to be noted that the delta is not wholly the product of a stream’s activity. The stream supplies the material, but the lake or sea renders at least passive assistance in its disposition. Not all rivers opening into the sea build deltas, and their failure is often the result of waves or shore currents which carry off the river sediment. Deltas are, however, sometimes formed in tidal seas, as at the debouchures of the Yukon; the Mackenzie, where the tidal range is three feet; the Niger, where the range is four feet; the Hoang-Ho, where the range is eight feet; and the Brahmaputra and Ganges, where the range is sixteen feet.[80] Since lakes, bays, gulfs, and inland seas have weaker waves and currents than the open sea, they are more favorable than the latter for the growth of deltas. Hence occur such deltas as those of the Mississippi, the Nile, the Po, and the Danube. Deltas are likely to be absent, or confined to the heads of bays, on coasts which have recently sunk. Their general absence on the Atlantic coast of the United States is a case in point.