The following figures give some idea of the extent of deltas, and of their importance in land building. The Mississippi delta is advancing into the Gulf at the rate of about 100 yards per year, or a mile in 16 or 17 years. Its length is more than 200 miles, its area more than 12,000 square miles, and its depth at New Orleans has been estimated at 700[81] to 1000[82] feet. This great depth is believed to be the result of subsidence, and so of the superposition of one delta on another.[83] The delta of the Yukon has a sea margin of 70 miles, and extends more than 100 miles inland. The delta of the Rhône has also had a remarkable growth, considering the size and the history of the stream. Arles, near the debouchure of the stream, was 14 to 16 miles inland in the fourth century +B.C.+, and is now 30 miles inland.[84] The Rhône has also built a great delta in Lake Geneva, and its lower delta is built of sediment gathered below the lake. The Po has built a delta 14 miles beyond Adria, the port which gave its name to the Adriatic Sea. The extension of this delta has been at the average rate of about 50 feet per year, but recently, on account of artificial embankments, the rate has been much more rapid.[85] The Ganges and Brahmaputra together have made a delta of great size. Its area is sometimes estimated to be as high as 50,000 or 60,000 square miles, and its head is more than 200 miles from the sea.[86] The head of the Nile delta is 90 miles from the sea, and it has a coastal border of 180 miles. The head of the delta of the Hoang-Ho is about 300 miles from the coast, and its seaward border has a length of about 400 miles, though with some highland interruptions.[87] After a delta has been built into a lake, the lake may disappear, leaving the delta out of water. Such “fossil” deltas, if so recently exposed that erosion has not destroyed their distinctive features, are readily recognized by their flat tops, their abrupt and lobate fronts, and their characteristic structure. They are often a means of determining the former existence of extinct lakes,[88] or the former higher levels of lakes which still exist.[89] Elevated deltas on seashores show either a rise of the land or a depression of the sea-level. The material which is carried along the coasts or shores from the mouths of rivers may take on various and peculiar forms, according to the strength, direction, and relations of waves and currents. The consideration of these forms belongs more properly to the work of the sea than to that of rivers, since rivers are not concerned in their construction except in supplying material. =Delta lakes.=—Delta-building streams sometimes help to form lakes by throwing their deposits around an area which fails to be aggraded to sea-level. Lake Pontchartrain, and other lakes in the delta of the Mississippi are examples (Fig. 187). [Illustration: +Fig.+ 192.—Terraces of the Frazier River at Lillooet, B. C. (Calvin.)]