152 ARNOLD AND ANDERSON— RECENT STREAM CONGLOMERATE 



solixble salts in the former blend with those in the waters of the latter, 

 producmg a mixture in which the magnesium forms such a small per 

 centum that if the cement is deposited it is of such a character as to be 

 less coherent and more easily redissolved during the rainy season than 

 that formed by White creek. Another reason why there are no deposits 

 of the conglomerate now forming along Los Gatos creek (and the same 

 conditions were probably prevalent in the past) is because along its lower 

 course the bed is largely aggraded and the water before it reaches the 

 point of saturation has passed into the underground channel. A prac- 

 tically impervious bottom, such as that of White creek where it flows on 

 the bedrock or else on the cemented conglomerate, is necessary to the 

 formation of the conglomerate, as it prevents the sinking of the water 

 into the underground channel before the point of saturation is reached. 

 It might be mentioned in this connection that wherever the base of the 

 conglomerate was seen it was resting on bedrock. 



A study of the phenomenon described in this paper makes it appear, 

 then, that the following conditions are prerequisite to the formation of 

 stream conglomerate of this kind : A small stream carrying in solution a 

 high per centum of magnesium carbonate, flowing over a rough, fairly 

 impervious bottom, with a gradient steep enough to keep the water in a 

 constant state of agitation, and a climate that is arid, at least during a. 

 considerable portion of the ye.ar, with consequent evaporation of such 

 rapidity as to produce a high degree of saturation in the stream before 

 it sinks beneath the surface or is diluted by the waters of some other 

 stream. 



Topographic Importance of the Deposits 



The conglomerate deposits of White creek have an important effect on 

 the topography in building up the valley bottom of an ungraded stream 

 and retarding the rate of erosion along the canyon bottom while erosion 

 continues to work on the rest of the drainage area. One effect of this 

 action is to raise the general level of White creek above that of the stream 

 to which it is tributary. The floor of the valley of the former is now 

 several feet above Los Gatos creek where the two join, and if the process 

 should continue and markedly affect the lower end of White creek, as 

 well as its middle portion, as at present it does not, the formation of a 

 hanging valley might result. It seems likely that the deposits are being 

 extended down stream, for the reason that when the pavement is formed 

 absoi-ption of water into the ground ceases and the stream confines itself 

 in a smooth, narrow channel, which allows of less evaporation and less 

 deposition of cement, and that therefore the length of the- course along 



