. Elements of geology : a text-book for colleges and for the general reader. Geology. DELTAS. 29 to the time, though many thousand years distant, when this lake will be entirely filled up. After leaving the lake the Eh one again gathers. Fig. 21. sediment from tributaries flowing in below the lake, and forms another delta where it empties into the Mediterranean. Many examples of lakelets partially filled, or entirely filled and converted into meadows, are found among the Sierra Mountains. In the section view (Fig. 21) we have represented the strata as irregular and highly inclined. This is cal
. Elements of geology : a text-book for colleges and for the general reader. Geology. DELTAS. 29 to the time, though many thousand years distant, when this lake will be entirely filled up. After leaving the lake the Eh one again gathers. Fig. 21. sediment from tributaries flowing in below the lake, and forms another delta where it empties into the Mediterranean. Many examples of lakelets partially filled, or entirely filled and converted into meadows, are found among the Sierra Mountains. In the section view (Fig. 21) we have represented the strata as irregular and highly inclined. This is called oblique lamination. This can only occur when a rapid stream, bearing abundant coarse material, rushes into still water. But, in the case of large rivers flowing long distances and bearing only the finest sediment, the stratification is much more regular and nearly horizontal. ^o^ Rate of Growth.—There have been several attempts to estimate the rate of growth of deltas, in order to base thereon an estimate of their age. The delta of the Rhone in Lake Geneva has advanced at least one and a half mile since the occupation of that country by the Romans ; for the ancient town Porta Valesia (now Port Valais), which stood then on the margin of the lake, is now one and a half mile inland. The delta of the same river at its mouth in the Mediterranean is said to have advanced twenty-six kilometres, or sixteen miles, since 400 b. c, or thirteen miles during the Christian era.* The delta of the Po has advanced twenty miles since the time of Augustus ; for the town Adria, a seaport at that time, is now twenty miles inland. But the most elab- orate observations have been made on the Mississippi. This river, as seen in Fig. 19, has pushed its way into the Gulf in a most extraor- dinary manner. According to Thomassy,f and also Humphrey and Abbot, the rate of advance is about one mile in sixteen years. The rate of progress in the deltas mentioned has, however, probably not been uniform. Th
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