16 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 Tributaries. 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 de/tas, 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 
