40 MASSACHUSETTS HORTICULTURAL SOCIETY 



caceous gneisses and others, both basic and acid, but including 

 many altered sedimentary beds. In the vicinity of Boston and 

 southward there are considerable areas of carboniferous sandstones, 

 conglomerates, and slates. 



The mantle of drift or till left by the glaciers masks and obscures 

 the pre-glacial peneplain. The most characteristic forms of the 

 glacial soils are the rounded hills of unmodified material known as 

 drumlins, which show a tendency to linear grouping. The peat 

 lands in their depressions are conditioned in the main by springs; 

 but unlike those located along fault lines and issuing from rock 

 crevices, the springs are intermittent and show their connection 

 with the seasonal rainfall. 



Another form of glacial drift are the plains of stratified sand 

 which appear to be chiefly deltas and outwash aprons formed by 

 streams during the successive stages of lowering of ancient glacial 

 lakes. These sand plains may be regarded as the natural reservoirs 

 of characteristically soft water, with a uniformly shallow water 

 table. Peat lands occupying depressions in the stratified drift are 

 well exemplified by the "Great Cedar Swamps," so abundant in 

 the southeastern portion of the state. The great extent and the 

 continuity of these sand plains has been one of the chief causes 

 in the obstruction of former drainage channels and in the formation 

 of those lake and river peat lands which today constitute the main 

 peat resource of the state. 



The till of the upland adjoining peat land is arenaceous, porous 

 in texture, and supplemented with occasional interbedded and 

 superficial layers of washed material. It readily absorbs the 

 greater portion of the rainfall and is quite susceptible to the " pod- 

 soling" process mentioned above. Differences in this feature may 

 be accounted for in part by the variations in the surface organic 

 materials, in the texture and character of sub-surface mineral soils, 

 and in the season's rainfall from year to year. 



The till underlying the peat lands is prevailingly well compacted 

 and probably is the principal source of supply as well as the factor 

 determining the nature of the ground waters. 



Beds of clay underneath peat lands are not numerous; they are 

 found deposited at relatively low levels of elevation, and to some 

 extent under the salt marshes and in the fresh water peat lands 

 near the coast. 



