96 SURFACE GEOLOGY. 



fifty years built up the largest city in the state. The plains on the west 

 side are extensive, both north and south of Piscataquog river, descending 

 from 280 to 265 feet above the sea. The upper edge of the alluvium in 

 the north-east part of the city is from 260 to 265 feet above the sea. 

 Elm street lies upon a terrace intermediate between the high plain and 

 that of the depot. Wells upon this street show about 25 feet of common 

 sand, in the lower part of which water is obtained by the shallower wells; 

 then, 20 feet of quicksand, with no springs, beneath which is a thin layer 

 of clay, lying on unmodified drift or till. The deeper wells find their 

 springs between the clay and till. 



Piscataquog village also lies mainly upon an intermediate terrace, which 

 extends a mile to the south, being occupied by the river road. The low- 

 est terrace appears between this and the river ; farther south it attains a 

 large width, and is crossed by the river road at Bowman's brook. The 

 same terrace is also wide on the east side north of Goff 's Falls. 



About a mile south from the city of Manchester an extensive, ledgy 

 hill, isolated in the alluvial area, lies between the Concord and the Man- 

 chester & Lawrence railroads. The plain below this barrier to Goff' s Falls 

 does not quite reach its normal height, being 210 feet above the sea, or 

 100 above the river at the mouth of Cohas brook. In the north-western 

 part of Londonderry it is slightly higher, and large quantities of its sand 

 have been blown to the south-east 75 feet above it into irregular mounds 

 and ridges upon the hillsides. Southward, the high plains are contin- 

 uous and wide to Hudson. 



On the west side the plains are wanting most of the way for six miles 

 from Bowman's brook in Bedford to Reed's Ferry. This side of the val- 

 ley appears to have been specially subjected to the erosion of the river, 

 which has in many portions removed nearly every trace of modified drift, 

 leaving the till or ledge sloping to the present channel. The till is fre- 

 quently exposed in the banks or bed of the river between Manchester 

 and Nashua, and also sometimes appears on the surface of the extensive 

 plains, showing that the modified drift has not so great a depth as in the 

 widest portions of this valley northward. Nearly opposite Goff's Falls, 

 and on Sebbins brook at the south line of Bedford, the alluvium expands, 

 forming three terraces, the highest in each case being 90 feet above the 

 river. 



