186 GEOLOGY OF THE NARRAGANSETT BASIN. 



of wear on a beach. The frequency with which this occurs in the larger 

 pebbles suggests that when submitted to the action of the waves the bits 

 were angular joint blocks, such as quartzites afford when broken out o\' a 

 cliff. (See photograph of one o\' these joint niches in pebbles, PI. XIV.) 

 Here and there an indented pebble may be seen. 



The Dighton group of conglomerates being the highest member in 

 this basin, the areas in which it occurs are, owing to the deep erosion of the 

 basin, somewhat removed from the present margin. The distance is gen- 

 erally from 3 to 4 miles. 



The topography of these conglomerate areas is bolder and more rugged 

 than that of the other recks in the basin outside of the Wamsutta scries. 

 The formation abounds in rounded, bare, rocky knobs, with steep vertical 

 bluffs facing outward from the synclinal axis over the area of the Seekonk 

 sandstones, again overlooking recesses in the formation itself. Long- rocky 

 ridges also abound, as in Dighton, Taunton, and Attleboro. now generally 

 given over to woods on account of the scarcity and infertility of the soil 

 and the genera] unsuitableness of the surface to agriculture. 



The conglomerate masses attain elevations of L50 to ISO feel above 

 sea level, or of SO to 1 00 t'eet above the surrounding level. In Swansea 

 the conglomerate ridges rise to 160 feet; in Dighton, to ISO: and in Attle- 

 boro, also to 180. Near Taunton, the Rocky Woods attain an elevation of 

 L60 feet. This level, indicated by the elevation of the conglomerate ridges, 

 is somewhere near that (^' the Jura-Cretaceous peneplain were it extended 

 eastwardlv over the Carboniferous area. The excavation o( the strata 

 below this level must be attributed to erosion in the Tertiary period. 



These conglomerate areas contain the headwaters of numerous brooks, 

 but several of the larger streams in the southern part o\ the held flow 

 across them, evidently from original courses which have been superposed 

 on these hard rocks. 



It remains to be determined whether there is an unconformity between 

 the Dighton conglomerates and the subjacent Seekonk group. It is to be 

 expected that even within the same geological period such discordances 

 would exist where currents were developed strong enough to urge cobbles 

 a foot or more in diameter out over an alluvial plain like that formed of the 

 Coal Measures. The original thickness ot the group is also a matter for 

 further investigation. Wore there higher Coal Measures in this area ! And 



