234 Longwell — Structure of the Triassic 



direction of faults recognized to the north and west (see 

 fig. 1) and of the great fault immediately southwest of 

 the ridges. Accordingly it is suggested that they are 

 depressions developed along fault zones. 



The fan deposit is in the upper part of the Triassic 

 section as it is known in Connecticut, and it forms the top 

 of the section at this particular locality. Its thickness 

 certainly is measured in hundreds of feet and may exceed 

 a thousand feet near the middle of the fan. The greatest 

 degree of coarseness and angularity occurs above the 

 upper or "posterior-" lava sheet, although typical fan 

 deposits continue to a much lower horizon. It appears 

 that the lava flow wedges out in the fan, for there is a 

 progressive thinning of the sheet eastward, and no out- 

 crops of it can be found in the area of coarsest and thick- 

 est fanglomerate. Fragments of basalt, many of them 

 vesicular and others quite compact, occur in large 

 numbers beneath the "posterior" sheet as well as above 

 it. The present dip of the beds is in a direction slightly 

 east of south and averages about 35°, although dips as 

 high as 45 c are found locally. 



The special interest of the fan lies in its bearing on the 

 structural history of the region. In this connection the 

 large size and extreme angularity of the fragments are 

 very significant features, testifying to the presence of a 

 steep scarp near the present border of the Triassic area 

 late in the period of sedimentation. Sediments thousands 

 of feet in thickness had already accumulated in the Tri- 

 assic trough, and without doubt much of this material was 

 stripped from the Eastern Highland during long con- 

 tinued erosion. The coarse detritus in the fan, therefore, 

 furnishes strong evidence of important faulting along or 

 near the line of the great boundary fault before the close 

 of the Triassic period. 



Abundance of vesicular basalt in the fanglomerate is 

 another notable feature. It is to be expected that the 

 lava flows would furnish some detritus before their burial, 

 but their widespread distribution in the Lowland and the 

 apparent absence of important unconformities in the 

 Triassic section indicate continued low relief during sedi- 

 mentation within the area where the flows now exist. 

 Moreover it is evident that the fan materials came from 

 the Eastern Highland, and therefore we must conclude 



