LARGER RELATIO^■S OF DELTAS K).") 



carried the stone for their buildings at a date estimated at thousands of 

 years ago (placed by Professor Bowman at probably 5,000 to 6,000 

 years) is still in existence, though in skirting around the south shore it 

 obviously kept close to the water. 



An excellent example of a delta which has been built into shallow 

 water of constant level is that of the Saint Clair Eiver.J Lake Saint 

 Clair has a maximum depth of 23 feet, and except on the side of the 

 delta the water deepens within a mile of the smoothly curved shore to 

 2 fathoms, the mark of the local wave base. The delta is built of fine- 

 grained waste derived by wave action on the shores of Lake Huron and 

 current action on the banks of the Saint Clair Eiver. Its front does not 

 show the crescentic reefs due to wave action. Shoal water of less than 

 1 fathom in depth extends outward from 2 to 4 miles from the main 

 area and from 1 to 2 miles beyond the mouths of the distributaries. The 

 lagoons thus arise from inclosure between growing distributaries rather 

 than behind a shore barrier. There is less water completely shut off, 

 however, than in the case of strong wave action. From the margin of 

 the shallow water the front slopes rapidly to 2 fathoms, the normal 

 depth of the lake. 



This forms a good measure of the limits of shoal water between the 

 land and the shore face in a delta built under conditions which make 

 this transition zone a maximum, since here the waste is fine, is not floc- 

 culated by salt water, and the wave action is weak. 



These marginal conditions of deltas built into shallow waters witli 

 weak wave action may be contrasted with those described for the iNile, 

 the Mississippi, and other large deltas built out against the oceans. In 

 earlier ages at stages of stationary crust and constant water level the 

 deltas of interior seas would build out until shallow protected waters 

 would disappear and they would in^'ade a region of constantly stronger 

 wave action. The sea would more and more tend to limit their growth. 

 Such a stage is now seen in the deltas facing the North Sea. Following 

 a movement of subsidence the contrary phase would occur; shallow 

 waters would pass far inland and the delta conditions would come for 

 the time to resemble those of the Yellow, Aral, and Caspian seas. Thus 

 there would be expected a cyclic oscillation in the character of the delta 

 front. 



The late Mesozoic delta cycle of the Atlantic Coastal Plain. — The de- 

 termination of the conditions of origin of a particular formation or part 

 of a formation depends in the first place on the study of the strata and 

 the application to them of detailed criteria; but after tliis is done the 



t L. J. Cole : The delta of the Saint Clair River. Geol. Siirv. Mich., vol. Ix, pt. I, 

 1903. 



