1877.] Surface Geology of the Merrimack Valley. 529 
The lower portions of these slopes were probably sixty feet be- 
low the surface of water, which was held back by the extensive 
plains brought in from the Contoocook and Soucook valleys. 
These plains have their greatest development on the east side of 
Merrimack River, extending six miles from above East Concord 
tothe mouth of theSoucook. Their areaof greatest width, which 
exceeds two miles, is opposite the city of Concord. 
In Boscawen and Canterbury and through Concord, the low- 
est terrace for twelve miles occupies a wide area, of which a 
large part is overflowed by the high water of spring, forming the 
only extensive interval on this river south of Plymouth, Fine 
views are here afforded at the edge of the plains, whose high 
bluffs descend abruptly a hundred feet, overlooking the fertile 
intervals and the windings of the river for miles north and south. 
Ancient river-beds are indicated at many places by shallow 
ponds, which lie in long and frequently curved depressions of the 
bottom-land. Horseshoe Pond is one of these, situated at the 
north end of Main Street, in Concord. It is shaped like a cres- 
cent, being a half mile long, nearly as wide as the present chan- 
nel, and six feet above the ordinary height of the river. Its 
middle portion lies at the foot of a higher terrace, against which 
the river once swept its full current. The nearest point of the 
present channel is a half mile distant at the north, where the 
river bends and now directs its current against Sugarball Bluff, a 
mile and a half northeast from Horseshoe Pond. The date of 
these changes cannot be stated; they occurred before the first 
settlement here, one hundred and fifty years ago. 
On the east side of the “ Fan” or broad interval opposite the 
north part of the city of Concord, the river formerly flowed by a 
very circuitous route four hundred and sixty rods, which was 
shortened to one hundred and fifty rods by great freshets, in 1826, 
1828, and 1831, cutting a direct course across two peninsulas. 
Ponds occupy portions of the old channel. Sugarball Bluff, one 
hundred and twenty-five feet in height, which forms the edge of 
the sand plain near this place, is now being rapidly undermined. 
At Davis’s Bluff, a mile to the south, and of about the same 
height as Sugarball Bluff, a width of three rods has been swept 
away in as many days. Erosion at this point has continued 
irty years, requiring a dwelling-house near the edge of the 
bluff to be several times moved and the road changed. 
These recent incursions of the river upon the plains, and the 
"niy changes in its channel upon the intervals, washing away 
o. 9. 34 . 
