240 



PHYSICAL GEOLOGY 



If these beds were traced from the shore outward it would be found 

 that, as in the diagram (Fig. 232), the sandstones projected as a thin 

 wedge between layers of shale. This " dovetailing " is due to tem- 

 porary changes in the conditions of sedimentation. During violent 

 storms sand or even gravel may be carried out much farther from 



Fig. 232. — Dovetailing of sediments. As sediments are traced from the shore 

 their character usually changes gradually. The dovetail structure is due to shifting 

 conditions; heavy storms carry coarse material to an unusual distance, and calm 

 weather permits the deposition of fine sediment close to shore. 



shore than usual, but when calmer weather prevails mud will be laid 

 down on the sand and gravels. During severe floods rivers also bring 

 down an enormous quantity of sediment which is swept a much 

 greater distance into the sea than normally. 



Basal Conglomerates. — If a coast is sinking more rapidly than it is 

 filled by sediments the shore will gradually retreat inland, with the 

 result that the beach of one period becomes deep water later. The 



Sea level 



I'- I V -I x ^ 



1 / 



/i'i''^ 



Fig. 233. — Basal conglomerate. When the old land surface (1) was slowly sub- 

 merged, gravel (2) was deposited along the shore and as a result of progressive sub- 

 sidence covered the ancient land surface. The strata (3), (4), and (5) were deposited 

 upon the gravel (basal conglomerate) as the distance from the shore increased. Con- 

 temporaneous deposits are shown by the dotted lines 3 and 4. 



con rse sediment of the submerged beaches will be covered by finer sand, 

 muds, and lime, the nature of the deposit depending largely upon the 

 distance from shore. These gravels which cover the old land surface, 

 when hardened (indurated), are called basal conglomerates (Fig. 233). 



