188 GEOLOGY OF THE NAEEAGANSETT BASIN. 



synclinal areas, and it is evident that these upper portions of the Carbon- 

 iferous have disappeared by erosion from this northern field. The rela- 

 tively gentle dips along this margin, together with the observed great 

 thickness of the formation where fully developed, would carry the outcrop 

 of the uppermost beds several miles south of the border and out of the 

 Dedham quadrangle. 



The structure of the strata in this area, so far as it can be made out 

 from observed outcrops, is mainly synclinal. The axis of this fold passes 

 approximately through Mansfield Junction and South Easton; or, in general 

 terms, lies at a distance of from 2 to 3 miles from the northern border. 

 The trough is broad and shallow toward Brockton, but its sides steepen 

 westward toward Mansfield, and between North Attleboro and the border 

 it is much compressed, and the folded strata are finally lost to view beneath 

 the block at Blake Hill. Its general features in the Mansfield area are 

 shown in fig. 27, on p. 190. The following observations set forth the evidence 

 in typical areas, about Mansfield and Bridg-ewater : 



MANSFIELD AREA. 



There are no surface exposures in the immediate vicinity of the West 

 Mansfield coal mines, the rock having been found in digging a well. The 

 lowest strata of the Carboniferous appear nearly 4 miles north of the coal 

 mines, dipping gently off to the south from the hornblendic granitite of 

 the Wrentham-Hingham uplift. The basal arkoses and grits, described with 

 the Wamsutta series (pp. 135-139), form conspicuous ledges on the southern 

 face of Foolish Hill, in Foxboro. The Wamsutta series has here a possible 

 thickness of 1,000 feet. It is succeeded on the south by quartz pebble 

 and quartzite conglomerates and gray sandstones, forming glaciated ledges 

 scarcely above the general surface of the glacial drift. Southward from 

 Foolish Hill toward Mansfield Junction, beds of sandstone and conglom- 

 erates appear, dipping about 25° S. A roche moutonne'e gives the follow- 

 ing' section from the top: 



Section of a roche moutonne'e in the Mansfield area. 



Feet. In. 



5. Sandstone, with layers of slate pebble conglomerate 6 6 



4. Sandstone 3 6 



3. Conglomerate, with slate pebbles 1 



2. Sandstone -' 5 



1. Conglomerate, base not seen 10 



