HAETLANDMONTVILLE SYSTEM. 



149 



the top, so that its cross sectiou is ahnost always arched. At Peltoma Point 

 the ridge crosses the Sebasticook River. The river can be forded on the top 

 of the ridge, but the water is much deeper on each side. It also rises 

 nearly to the surface while crossing Unity Pond. The ridge broadens 

 south of Unity Pond, and from near Unity Village a plain of complicated 

 structure extends south along the valley of Sand}^ Stream almost to Thorn- 

 dike station. The plain fills the valley from side to side, and is from one- 

 fourth to one-half of a mile wide. It shows some arched ridges of gravel, 

 bordered and often covered by a more nearly horizontally stratified stra- 



riG. 16.— Osar; Pittsfleld. 



tum of fine gravel, sand, and clay. Originally there were kettleholes, but 

 most of them have been filled or nearly filled by the later sediments. The 

 sea certainly extended to Unity, as is proved by marine fossils. How far 

 it extended up the valley of Sandy Stream is uncertain. The contour of 

 230 feet would be found 1 or 2 miles south of Unity Village. The origin 

 of this plain will be discussed more fully later. 



Not far from the junction of Sandy Stream with Half Moon Stream 

 the gravel comes up out of the valley. For a half mile southward it takes 

 the form of a broad osar, or perhaps delta-plain. Then for several miles it 



