GLACIAL DEPOSITS. 29 



percolating waters. As elsewhere stated, this roofing slate resists weather- 

 ing to a remarkable degree. All the circumstances make it certain that so 

 great an amount of weathering as is shown by the slate in the bottom of 

 this depression could have been accomplished only in the long eons of pre- 

 glacial time. The bronwish mass in the bottom of the depression is a 

 residual earth, a soil of preglacial weathering. This subject will be referred 

 to hereafter. 



GLACIAL DEPOSITS. 



THE TILL. 



Resting upon the glaciated rock (or here and there upon the small 

 areas of uonglaciated rock weathered in preglacial time) is the till. It is 

 an endless study. So varied are its forms and developments that no 

 attempt can be made within the space allotted to this portion of our subject 

 to do more than refer to those properties especially related to the subject 

 of the glacial gravels. At the present time we do not need to theorize 

 concerning the existence of a great body of laud ice over northeastern 

 North America. Assuming that the area of Maine was covered by a series 

 of ice fields that were practically confluent, so as to form an ice-sheet, we 

 interpret the facts as to the till in accordance with the glacial hypothesis. 



The names given to the till in Maine deserve notice. A very common 

 name for the formation is "hardpan." This no doubt refers to the compact- 

 ness of the formation and the difficulty of digging into it. Another common 

 name is "pin gravel," though the same name has also been applied to any 

 recent conglomerate or water-washed gravel cemented into a firm rock by 

 carbonate of lime or by iron oxides or hydi-ates. The till usually contains 

 many stones and bowlders of all sizes, and a soil composed of weathered 

 till is conunonly known as "hard, rocky land," or as "rocky, ujjland soil." 

 It is often called "hard-wood soil," also "orchard laud." It is unfortunate 

 that the term ' ' gravel " is so often associated with the till. In Maine when 

 soil is described as "gravelly," in most cases it is meant that the soil is 

 composed of till. "Gravelly loam" almost always means till, but some- 

 times it means a thin stratum of marine clay overlying and partially mixed 

 with true water-assorted and rounded gravel. Many know the formation 

 as the "bowlder clay." To apply the terms "gravel" or "clay" to the till is 

 a fruitful source of confusion, causing the till to be confounded with water- 



