10. TIDES AND CURRENTS 



Ocean tides enter the bay throup;h the East and West Passa,5es 

 and the Sakonnet River, Two approximately ecrual tides occur each 

 day, a tidal cycle averaging 12. U hours, Kean tide ranges from 

 3,^ feet at Newport to U.6 feet at Providence and spring tide 

 from. k,h feet at Mex-jport to 5.7 feet at Providence, Tidal nove- 

 m.?nt is nearly simultaneous throughout the bay,, high and low tide 

 for most points occurring tri-thin twenty minutes of high and low 

 tide at We-ixport, High water at Providence occiirs about 10 minutes 

 after high I'jater at Iiei-Tport, The usual maximum flood or ebb 

 current is from 0.5 to 1,0 knotj however, in narrow areas of the 

 bay, the current is as much as 2,75 knots as at the Sakonnet River 

 bridges. For spring tides, the usual maximum velocities are about 

 20 percent greater than the above values. 



11. GENrSAL GEOLCfiy AW TOPOGRAPHY 



a. Structure . The bedrock of the eastern and southeastern 

 seaboard lowland of New England is predominantly gneissic and 

 granitic, except where complicated by the presence of schist, as 

 in the case of southwestern Rhode Island, or xjhere a low spot in 

 ;bhe crystallines contains residual 7^ounrer sedimentary rocks. 

 Many of the j.gneous rocks have been attributed to the Devonian 

 Period, wliile rocks in the two most prominent depressions, the 

 Boston Basin and the Narragansett Basin, are largely carboniferous; 

 thus the entire area is predominantly lo^rer to middle Paleozoic 



in age. 



Glaciation and stream erosion were much more effective in the 

 sedimentary rocVs of the basin than in the surrounding crystallines, 

 and caused the entire basin to exist as a depression. Narragansett 

 Bay, hoTvever, occupies merely the southvjectern portion of the 

 basin, and represents the surface flooding of some of the deeper 

 valleys of the de-nression. Attempts have been made to visualize 

 distinct structures in the basin. The rocks have been described 

 as dipping steeply in a series of broken synclines and a rough 

 estimate of two miles has been ventured for the tlaickness of 

 Carboniferous strata represented in the basin, but the inter- 

 pretation involved postulation of numerous faults, and appears 

 somewhat contrived. 



b. The rocks of Narragansett Bay . The rocks of the bay are 

 largely sandstones and shales, sometimes coaly, as in the deposits 

 of the old "coal" mine north of Newport. Mineral alteration is 

 quite pronounced locally, presumably owing to adjacent granitic 



contacts north of the entrance to J-ount Hope Bay, Ne>:port Head, 

 the southern tip of Jamestowi Island, and the coast of Narragansett, 

 the latter granite having been established only rather recently as 

 a yoTinger intrusional body. 



