70 PROCEEDINGS OF THE CHICAGO MEETING 



GEOLOGICAL NOTES ON PALESTINE 

 BY R. W. BROCK 



(Abstract) 



The rocks range from pre-Cambrian to recent, but the country is underlain 

 largely by Cretaceous limestones and recent basalts. 



The main structural features are faulting; the Judean hill country is an 

 asymmetric fault-block. Fault-blocks usually sink without much tilting, but 

 the Jebel Usdum block is upturned. 



The topography is dependent on structure, even down to details. Some of 

 the volcanoes at least have been extruded from faults. The Jordan trench 

 was filled to a height of about 1,400 feet above the present level of the Dead 

 Sea during a pluvial period, perhaps coincident with the glacial. There has 

 been no marked warping since. 



The prepluvial canyons of the rivers Arnon and Zerka Main indicate a cli- 

 mate slightly drier than the present and a rapid change to pluvial conditions. 



The beds laid down by the disappearing Jordan Lake were formed within a 

 period of less than 30,000 years. 



The salts in the Dead Sea that have not yet reached the saturation point 

 would be accumulated in less than 50,000 years. 



Geological evidence points to a dry climate ever since the pluvial period. At 

 present the climate is a little less arid than formerly. 



Presented from notes and with the aid of lantern slides. 



Discussion 



Prof. A. C. Lawson : Modern earthquakes in the region of the valley of the 

 Jordan show that the movements which gave rise to the graben are still going 

 on. I should like to ask if the old lake beds of the valley show the effects of 

 faulting, or whether the valley which traverses them is due to erosion as the 

 waters of the lake fill. 



The author replied to Doctor Lawsou : The lake beds show very slight faults, 

 but the striking feature is their undisturbed condition, notwithstanding that 

 movements and heavy earthquakes are still taking such as that which de- 

 stroyed Tiberias. 



To a query regarding the occurrence of petroleum, the author said : There 

 are a few oil seepages in the Nubian sandstone, on the east shore of the Dead 

 Sea, from the lower beds of the Upper Cretaceous toward the southeast end 

 of the Dead Sea. The floating asphalt indicates seepage from the fault in the 

 bed of the south lagoon. 



FINAL STAGES IN THE PHYSIOGRAPHIC EVOLUTION OF THE SAN JUAN 



MOUNTAINS > 



BY WALLACE W. ATWOOD 



iAJjstract) 



The field studies on the physiography of the San Juan Mountains have been 

 reported on from time to time at the meetings of this Society. As the studies 



' Presented with the permission of the Director of the U. S. Geological Survey. 



