136 



CAPE COD GEOLOGY 



LANDSLIDES 



At several places in the cliffs there may have been small landslides, which 

 have involved beds of clay, probably when wet. The attitude of a bed may 

 determine its stability; a bed in one position is more likely to slip than a bed 

 in another. (Plate 20, fig. 2.) 



STRATIGRAPHIC GEOLOGY 

 Introduction 



OUTSTANDING GEOLOGIC FEATURES AND OBSTACLES TO 



GEOLOGIC WORK 



NATURE OF THE DEPOSITS 

 The deposits on Marthas Vineyard are unconsolidated beds laid down in 

 late geological time. At many places they are greatly distorted, and they cover 

 and hide all older Pleistocene beds. For this reason the boundaries of forma- 

 tions cannot everywhere be determined accurately, and at some places they 

 have been drawn where the relief changes. 



THE TOPOGRAPHIC MAP 

 The topographic base map used was made in 1887, when it was not con- 

 sidered necessary to represent the features in as great detail as is now essential 

 for detailed geologic work. At many places the contouring was so greatly 

 generalized that the character of the surface is entirely lost. Furthermore, 

 the map is in some respects inaccurate. During the forty years that have elapsed 

 since the map was made, there have been many changes, some due to natural 

 causes, and some due to the work of man. 



THE WISCONSIN DRIFT 



The sheet of Wisconsin drift mantles all older deposits. In the morainal 

 areas it is usually a sandy till, which at some places is as much as 6 feet thick. 

 In the great plain it consists entirely of a thicker covering of outwash gravel. 

 In the till-covered area the drift has only in part obliterated the older surface. 

 In the outwash plain it has completely buried the older surface. The task of 

 determining the location and character of the older beds in the morainal areas 

 is therefore difficult, and in the outwash area it is at most places nearly or 

 quite impossible, 



