256 



flat top of the delta dug out the groove for a few feet, but was deflected 

 upward and spread out into a thin sheet before reaching the edge. This 

 interpretation was confirmed by the records of rainfall and lake level kept 

 by the Rochester water works at the foot of the lake, which is the source 

 of public supply for that city. These records are as follows : 



Dale. 



Hours. 



Rainfall. 



Lake Level Above Datum. 



July 5 



" 6 

 " 7 

 " 8 



12:30-7:00 p. m. 

 In the night. 

 10:00-11:00 p. m. 



.921 in. 



2.349 " 



.546 " 



.101 " 

 1.397 " 



.607 " 

 .864 " 



1.736 ft. 

 2.926 " 

 3.106 " 

 3.126 " 



" 18 

 " 19 



" 20 

 " 22 



8:00-11:00 a. m. 



In the night. 

 2:00-6:00 p. ra. 

 In the night. 



2.176 " 

 2.296 " 



2.546 " 

 3.186 " 









These deltas were begun during the heavy rains of July 5-7, when 

 3.S16 inches of rain fell and the lake rose 1.39 feet, most of the work being 

 done in the night of July 6, when 2.35 inches of rain fell. They were com- 

 pleted July 18-20, when 2.97 inches of rain fell and the lake rose 1.01 feet. 

 These miniature torrential deltas furnish suggestions for the interpreta- 

 tion of similar but larger features which mark the shore lines of the tem- 

 porary glacial lakes formerly occupying the Finger lake valleys. 



A similar flat-topped, steep-sided feature caught the writer's eye on 

 the east side of Honeoye Lake. Projecting from the steep hillside like a 

 bracket it rose 200 feet above the lake, suggesting by its bold and sym- 

 metrical outlines an artificial origin similar to that of the dump pile of ;i 

 mine (Fig. 2). It proved to be a torrential delta built at the mouth 

 of Briggs gull. Its finely curved front slope, about 150 high, is as steep 

 as the material will lie. Its flat top is traversed by a channel twenty feet 

 wide and three feet deep which extends to the edge and is continued by a 

 similar groove in the steep face. The southern side cut away by the main 

 stream shows characteristic foreset beds of sand containing large frag- 

 ments of shale near the top. Briggs gull now drains a basin of about six 

 square miles. A heavy rain with rapid melting of ice or a sudden diversion 

 of drainage by the breaking of an ice dam in glacial times may have en- 

 abled the stream to build this delta in a few days or weeks. Briggs delta 

 helps to account for the anomalous distribution of glacial lake deltas. 

 Similar features are numerous in the Finger lake valleys. Not their 

 presence but their absence from the former mouths of many streams seems 



