?55 



Some Features of Delta Formation. 



By Charles R. Dryer. 



In August and September, 1902, the writer spent some weeks among 

 the western Finger lakes in Livingston and Ontario counties, New York. 

 Along the shores of Hemlock Lake his attention was attracted by many re- 

 cently formed deltas which seemed to present unusual features. Each 

 delta was a semi-circular pile of fine shale shingle symmetrically arranged 

 around the mouth of a little gully formed by a wet weather stream. The 

 level top stood about two feet above the lake surface and was bounded by 

 a bank of shale which sloped downward about three feet in six to a mud 

 line under water. The wash of waves had cut at the top of the slope a 

 vertical cliff six inches high. The land side was bounded by a very steep 

 bank of stratified shale, a por- 

 tion of the general lake shore, 

 which is almost everywhere pre- 

 cipitous. From the month or' 

 the gully a groove a foot wide 

 and six inches deep extended 

 straight out half way or more 

 across the top of the delta, but 

 in no case reached the water's 

 edge. Along the sides of the 

 groove lay sticks of wood and 

 fragments of shale of relatively 

 large size. One medium sized 

 delta measured thirty-one feet 

 by twenty-six in diameter. No 

 camera was at hand, but sketches were made from which a rough model 

 was constructed and photographed. (Fig. 1.) 



The interpretation of the phenomena seemed plain. These deltas were 

 built during an exceptionally violent storm which filled the gully with a 

 rushing torrent and raised the level of the lake. The force of the stream 

 was abruptly checked at lake level and its load was deposited in the form 

 of a fan-like delta. Toward the last of the storm the stream striking the 



Pig. l. 



Model of Torrential Delta in 

 Shale Gravel. 



