l6 SURFACE GEOLOGY. 



eighth mile to one mile, leaving numerous terraces at each side. The 

 Merrimack and Saco valleys show similar erosion, and it may be seen 

 upon a small scale on every river in the state. On our largest rivers we 

 see the highest plain in some places, and the lower terraces very fre- 

 quently, being now undermined by the wear of the current, forming steep 

 bluffs and banks. It seems impossible to explain in any different way 

 the cause of the slope, often nearly as steep as is possible for loose mate- 

 rials, which forms the abrupt face or escarpment of level-topped and 

 horizontally stratified terraces. The finer character of the materials 

 which compose the lowest terraces and the interval, or present flood- 

 plain, is due to this wearing away and re-deposition by the river, which 

 have been many times repeated, till what may have been at first gravel 

 becomes very fine sand or silt. By each removal it is made one degree 

 finer, and is deposited at a lower level and farther down the stream. The 

 end of its slow journey is the sea, where it will help to make the sedi- 

 mentary rocks of this epoch. It has completed a great cycle of changes, 

 ending where the upheaved and contorted ledges from which it was de- 

 rived had their remote beginning. 



Deltas of TribiUaries. Upon entering the large valleys, tributary 

 streams of comparatively narrow channel and rapid descent frequently 

 formed extensive deposits in the Champlain period, similar in material 

 to the flood-plain of the main valley, but having a greater height. Some- 

 times these deltas, being partially undermined, form conspicuous terraces 

 a hundred feet above the highest normal terrace, which is the remnant of 

 the river's continuous flood-plain. The deposition of the modified drift 

 of the main river was usually but not always to the same level across the 

 valley. The increased supply from tributaries was sometimes a tempo- 

 rary barrier, damming up the waters of the main valley above ; and the 

 current could then deposit its sediment principally upon one side, making 

 the highest normal terraces quite different in elevation. 



Dunes. Wind-blown banks of sand or dunes, apparently isolated on 

 the hillsides, are occasionally found along the east side of Connecticut 

 and Merrimack valleys and south-east of Ossipee lake, at heights vary- 

 ing from the level of the highest terrace or plain to two hundred feet 

 above it. These patches of sand are very conspicuous, because they are 

 often destitute of vegetation, being blown in drifts by the wind. They 



