98 SURFACE GEOLOGY. 



being interval, and the last the high plain. They all extend southward 

 beyond the village, except the second, which terminates a short distance 



Fig. 22. — Section in Merrimack and Litchfield through Thornton's 

 Ferry. Length, 34 miles. 



south-west of the depot. The third represents the immediate terrace 

 which we noted as commencing at Reed's Ferry. The lower terrace 

 along this distance is now in large part above the reach of the annual 

 floods ; but its undulating surface, very noticeable along the railroad, 

 shows that nearly every portion of its area has been at some time occu- 

 pied by the constantly varying channel of the river. The crescent-shaped 

 pond north of Naticook brook lies in an ancient river-bed ; another an- 

 cient channel, considerably above that of the present day, is crossed 

 shortly after turning off from the main road in Litchfield to go to Thorn- 

 ton's Ferry. At the east landing of this ferry the bottom of the bank is 

 a thick stratum of clay, which is overlaid by sand. 



Through Nashua we find the width of the alluvium narrowed, and till 

 extending at several places almost to the river. An isolated area of till 

 lies close to the railroad just south of Pennichuck brook. A former 

 channel of this brook is plainly traceable here for a mile ; it is crossed by 

 the railroad a short distance south of the bridge, and thence extends 

 southward, forming a long, nearly straight hollow in the terrace between 

 the railroad and the river. A short distance farther south a succession 

 of four terraces appears, at heights of 30, 55, 65, and 95 feet above the 

 river. The highest of these forms a plain, over which the road next to 

 the river extends for a mile south from Pennichuck brook. A small peat- 

 bog lies in a depression on the west part of this plain. Two thirds of a 

 mile north of Nashua river a narrow area of till extends almost to the 

 Merrimack. Much of the till of this section is quite different from that 

 usually seen, as it contains very few large boulders : its coarser portions 

 are mainly pebbles and chips of rock, not often exceeding one foot in size. 

 The former were derived from the neighboring Lake gneiss at the north, 

 and the latter from the compact mica schist and quartzite of the Mer- 



