340 J. C. Russell— The Jordan-Arabah and the Dead Sea. 
The date of the beginning of this independent drainage system 
is not definitely known. It was post-Cretaceous, for the reason that 
Cretaceous rocks form its walls. That it existed as a prominent 
topographic feature previous to the Glacial epoch is shown by its 
lacustrine records, taken in connection with the history of adjacent 
regions during the Quaternary period. Its birth must therefore 
have been during the Tertiary period. 
The best estimate that can be made at present of the extent of 
the hydrographic basin of the Dead Sea shows that it occupies 
an area of between nine and ten thousand square miles. The Dead 
Sea is about 274 square miles in extent.? Consequently the ratio of 
lake surface to drainage area is about as one to thirty-three. The 
lowest point in the basin is depressed 2570 feet below the level of 
the Mediterranean, and the lowest point on its rim is 285 feet 
above the same datum plain, or 1577 feet above the surface of the 
Dead Sea. It is evident from the present topography that this 
hydrographic basin must have held a lake in its lowest depression, 
either continuously or at intervals during humid periods, ever since 
it was cut off from oceanic drainage. 
A lake occupying the Jordan-Arabah depression, like all inclosed 
lakes, must have been sensitive to climatic changes, and could its 
past fluctuations be determined in terms of climate, they would 
furnish a geological weather record of unusual value. In illustration 
of this it may be observed that as the present arid climate of the 
Dead Sea basin admits of the existence of a large lake in its lowest 
depression, it follows that only during periods of excessive aridity 
could the ratio of evaporation to precipitation be increased suffi- 
ciently to cause the lake to disappear. 1f we could show, therefore, 
that the Dead Sea was evaporated to dryness, or was greatly con- 
centrated, at any period in its history, we should have proof of the 
former prevalence of an unusually arid climate in Palestine and 
adjacent regions. On the other hand, could we show that the Dead 
Sea once had a much greater expansion than at present, it would be 
conclusive evidence of a former period of greater mean bomidney 
than the region about it now enjoys. 
The fact that the Dead Sea basin has been long isolated is in 
itself sufficient to suggest that it has an interesting and perhaps 
highly complex lacustrine history. Fortunately, however, we have 
observation as well as hypotheses to help us in this connection. 
From the reports of many explorers we know that the shores of the 
Dead Sea and the borders of the Jordan valley are scored with 
terraces, and that lacustrine sediments, in some cases charged 
with the shells of fresh water mollusks and at other times inter- 
laminated with layers of salt and gypsum, are found over a very 
large area. Sufficient facts of this nature have been reported by 
competent observers to show that the Dead Sea basin has a record 
' Hull places it at the close of the Eocene. Geol. and Geog. of Arabia Petrea, 
Palestine, etc., p. 108. 
2 Determined from the map accompanying the narrative of the U.S. Expedition. 
The mean depth of the Dead Sea as obtained from the same map is 500 feet. 
