CONCRETIONS IN STREAMS FORMED BY THE 



AGENCY OF BLUE GREEN ALG^ AND 



RELATED PLANTS. 



By H. JUSTIN RODDY, M.S., Ph.D. 

 {Read May 7, 191 5.) 



In 1898, I discovered that concretionary formations occurred 

 in Little Conestoga Creek, Lancaster County, Pa. At that time, 

 however, I was engaged in other studies and gave the concretions 

 only a passing notice. But in the late summer of 1914, my atten- 

 tion was directed to the subject again by the reading of Dr. Wal- 

 cott's paper on "Pre-Cambrian Algonkian Algal Formations " which 

 appeared July 22, 1914. This paper made me realize the impor- 

 tance of a careful investigation of these particular stream forma- 

 tions as to characteristics, distribution, origin, etc. I began at once 

 a careful and extended search in the Little Conestoga as well as in 

 other streams for concretionary structures of recent formation. 

 My search was amply rewarded by finding them in great quantities, 

 and distributed throughout nearly the entire length of the Little 

 Conestoga. I found also that they not only occur in the creek 

 itself, but that quite large deposits of the concretions underlie the 

 flood plain meadows along the creek banks. One of these in Ken- 

 dig's Woods, two miles southwest of Millersville, Pa., is made up 

 wholly of concretionary materials on the top of which forest trees 

 of large size and considerable age are growing. This deposit 

 covers nearly an acre to the depth of about 8 feet in the middle 

 thinning out lenslike toward its edges. Another deposit along the 

 same stream near Fruitville in Evan's Meadow, more extensive in 

 area but of slighter depth, forms a substratum under a thick soil 

 cover and has an average depth of about two feet. Deposited con- 

 cretions occur under similar conditions in many other of the 

 meadows along the stream as is shown by weathered concretions 

 occurring in the soil and wash wherever wet-weather stream gullies 

 have been torn through the soil cover. 



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