94 



TEEKACES. 



they were, after careful study and protracted investigation, will be in general adopted in 

 this Report. In the memoir referred to, facts are presented from Europe, Asia and 

 Africa, as well as from the United States, from which to draw conclusions. So in drawing 

 our conclusions in the present Report respecting the formation of terraces and sea beaches, 

 we may be constrained to infer from phenomena not noticed in Vermont, or else existing 

 under forms scarcely recognizable, when considered by themselves alone. Still, the most 

 prominent objects worthy of notice in our discussion are found in Vermont, and will be 

 given in detail in the following pages. 



The term terrace applies to any level-topped surface with a steep escarpment, whether 

 it be solid rock or loose materials. We apply the term, when treating of surface geology, 

 only to those banks of loose materials, generally unconsolidated, which skirt the edges of 

 the valleys about rivers, ponds and lakes, and rise above each other like the seats of an 

 amphitheater. There is no danger of confounding the alluvial terraces (of which alone 

 we speak) with terraces of the tertiary period, unless it be a mongrel terrace of the later 

 tertiary, found along the west base of the Green Mountains. This will be treated of 

 under tertiary rocks. 



GENERAL LITHOLOGICAL CHARACTER AND RELATIVE SITUATION OF THE 



TERRACES AND BEACHES. 



In general, the following description is applicable to the terraces and ancient beaches : 

 1. The most perfect terrace is an alluvial meadow, annually more or less overflowed 

 and increased by a deposit of mud or sand. Except in rough mountain streams the 

 materials are rarely as coarse as pebbles over an extensive surface, and they are distinctly 



stratified. (See A on Fig. 39, which is an. 

 ideal section across a terraced valley.) 



2. Ascending to a second terrace, we almost 

 invariably find it composed of coarser mate- 

 rials. In the section, it is represented (Fig. 

 39, B) as composqd of clay beneath and sand 

 or fine gravel above. This is an arrangement 

 common to the greater part of the second 

 and third terraces throughout Vermont and 

 New England, though sometimes the sand is 



Ideal Section of a Terraced Valley. wantinP" 



3. The third terrace, or the next above the clay and sand terrace, is usually a mixture 

 of sand and gravel ; the latter not very coarse, the whole imperfectly stratified, and also 

 sorted ; that is, the fragments in each layer have nearly the same size ; as if the waters 

 that removed and deposited the materials, had a different transporting power for each 

 stratum. (C, Fig. 39.) 



4. The fourth terrace or the highest, D in Fig. 39, differs from the last by its coarser 

 materials and more irregular surface. As we ascend the terraces from the river, 

 we find the character of each less distinct. For the highest terraces were first 

 formed, then the next lowest in succession. Hence rains and freshets have acted the most 

 upon the upper terraces, and have caused these irregularities. Indeed the lower terraces 



Fio. 39. 



