82 SURFACE GEOLOGY. 



near East Concord, the river has changed its entire width from south- 

 west to north-east, and a third of a mile to the east it has changed more 

 than its width in an opposite direction. On the east side of the "Fan" 

 or broad interval opposite the north part of the city, the river flowed in 

 1804 by a very circuitous route 460 rods, which was shortened to 150 

 rods by great freshets in 1826, 1828, and 1831, which cut a direct course 

 across two peninsulas then known as Sugar Ball point and Hale's point. 

 Ponds already mentioned occupy portions of the old channel. Ten years 

 later, Dr. Prescott reported the rapid undermining of Sugar Ball bluff, 125 

 feet high, of which the river had carried away, between 1853 and 1863, a 

 mass 80 rods long and 40 rods wide. This erosion is still going forward, 

 being aided by springs near the foot of the bluff. At Davis's bluff, about 

 a mile south, a width of three rods was swept off in 1 863 in three days. 

 Erosion at this point has continued thirty years, requiring a dwelling near 

 the edge of the bluff to be several times moved, and the road changed. 



The same undermining of the high plains by the river is also going on 

 at several places north and south of Fisherville. One mile south-east 

 from Boscawen bridge, the plain, no feet above the river, is fast wear- 

 ing away, and portions of it 10 feet wide and 150 feet long had fallen 

 in 1875 10, 20, and 40 feet, remaining nearly level, so that their sapling 

 pines, 10 to 30 feet high, were still upright and growing on the side of 

 the steep sand-bluff. These would be carried away, to be followed by 

 new slides during the next high flood. One mile farther south, and at 

 other points below Fisherville, a similar rapid erosion was observed. A 

 quarter of a mile north-east from Fisherville bridge, a bluff, which has 

 been so recently undermined that it is not yet grassed, is now separated 

 from the river by a wide area which does not exceed five feet above the 

 ordinary height of water. These recent incursions of the river upon the 

 plains, and the rapid changes in its channel upon the intervals, washing 

 away yearly from one bank and adding to the side opposite, leave no 

 doubt that the river has flowed at the foot of the bluffs along their whole 

 extent, occasionally making a deep excavation beyond its ordinary bounds, 

 as on the east side south of Sugar Ball bluff; that the high plain once 

 filled the whole valley; and that the river has swept many times from 

 side to side over the space occupied by its lower terraces and interval. 



Important changes in the channel of the Merrimack have also been 



