POSTGLACIAL DEVELOPMENT OF CONNECTING RIVERS OF GREAT LAKES. 



483 



Twenty-nine of Cole's thirty-one borings are on the new delta, and one is on or about on the 

 dividing line between the new and old. Only one is distinctly on the old. Six borings pene- 

 trate muck beds, and five of these are on the new delta. One is on the North Channel at Point 

 aux Trembles, three on the Chenal a Bout Rond (two at one place), and one on the Middle Chan- 

 nel. The sixth is on the west side of the Bassett Channel, about half a mile south of its head, or 

 about on the dividing line. The one boring on the old delta is east of the dividing line a little 

 below the head of the Blind Channel. It disclosed deep muck at the surface but no buried 

 muck or hardened lake clay. 



It is inferred that all the buried muck beds found, with the possible exception of the one 

 on the Bassett Channel, date from the discharge of the upper lakes through the North Bay outlet. 

 The muck of the boring on the Bassett Channel may be of the same origin. At any rate, it can 

 hardly be taken as sufficient proof that buried muck and an old land surface exist under the old 

 part of the delta, and that the delta holds similar records of the discharge of the upper lakes 

 through the Trent Valley outlet. The boring on the Bassett Channel is offset bj~ that on the 

 Blind Channel, in which no buried muck or hardened clay was found. The substratum of the 

 old delta remains to be investigated. 



The flow of St. Clair River before the opening of the Trent Valley outlet was probably too 

 brief for important delta growth. In that event the only important period of delta growth before 

 the present one was during the time of the third or Port Huron- 

 Chicago stage of Lake Algonquin. The level of St. Clair River 

 was at that time higher than now and the delta was built to a 

 higher level. The old dry distributary which runs south from 

 2 miles north of Port Lambton suggests a delta head perhaps 

 as far north as Marine City and at a higher level. The delta 

 head near Algonac, with surface 6 feet above the present river, 

 seems related more nearly to the time of the Nipissing beach 

 (transition phase), which was the beginning of the modern 

 lakes. It stands close to that level. 



Since the closing of the North Bay outlet St. Clair River 

 has flowed continuously as the outlet of the upper lakes. If 

 the old and new parts of the delta correspond to definite parts 

 of the lake history the new part was probably made since the 

 closing of the North Bay outlet and the old part during the 

 southward discharge (Port Huron-Chicago stage) of Lake 

 Algonquin. 



In discussing the history of delta growth, Cole makes use 

 of one factor which has since been eliminated. He estimates 

 the effect of the northward tilting of the land which is supposed to have continued through 

 recent times to the present and to be still progressing and discusses its effect on the supposition 

 that the delta and Lake St. Clair lie within the area of the progressing uplift. The idea is an 

 application of conclusions reached by Gilbert, 1 in a study of recent and present land movements 

 in this region. It has since been found, however (pp. 439, 449) , that the hinge line of the uplif ted 

 area passes across the "thumb" and the south part of Lake Huron, so that St. Clair River and 

 Lake St. Clair with the delta are not affected by recent deformation or tilting. 



Figure 12.— Map of Point Edward spit. 



GRAVEL SPIT AT HEAD OF ST. CLAIR RIVER. 



A gravelly deposit, here called Point Edward spit, lying at the head of St. Clair River, on 

 the Canadian side (see fig. 12), is a distinct correlative of the newer part of the St. Clair delta 

 (p. 484). It has played an important part in the history of the river and has produced a 

 marked effect on the American shore. Studies of this locality still lack much of completeness, 

 but the general relations of the deposits have been determined. 



i Gilbert, G. K., Recent earth movements in the Great Lakes region: Eighteenth Ann. Rept. U. S. Geol. Surrey, pt. 2, 1S98, pp. 595-647. 



