﻿GEOLOGY OF THOUSAND ISLANDS REGION 139 



that district since the ice left that locality. The Gilbert plane 

 declines to the south and southwest and on the south border of 

 our area the beaches are 390 feet [pi. 45]. North of Lafargeville 

 [pi. 46] strong beaches lie at 440 feet, and 2 miles southeast of 

 Redwood a bar is found at 450 feet altitude [pi. 47]. 



It has not seemed practicable to make maps for this writing 

 to show all the Gilbert shore lines of the area, but the strongest 

 shore features are indicated on the maps, plates 45-47. These 

 are wave-built bars and spits and wave-washed limestones. Some 

 of these features are shown in the halftones, plates 48-53. The 

 southeast portion of our area, being the southeast diagonal half 

 of the Theresa sheet, was mostly above the Gilbert waters. The 

 submerged parts are such as lie below 400 feet at the south edge 

 of the sheet and below 440 feet at the north edge. It will be 

 seen that this is the low ground north and northeast of Brown- 

 ville, the valley of Perch river, the low ground about Theresa 

 and the valley of Indian river. All the rest of the region was 

 under the full Gilbert level except the three limestone hills north- 

 west of Dexter; the limestone plateau between Stone Mills, De- 

 pauville and Lafargeville ; the limestone plateaus north of Depau- 

 ville; the boulder-kame hill 2 miles north of St Lawrence cor- 

 ners, known as • the " Hogback " ; and the group of boulder- 

 moraine hills north of Lafargeville; one being cut by the edge 

 of the Theresa sheet. These areas which received wave action 

 so as to leave beach records are mostly shown in the plates 

 45, 46 and 48. 



While all surfaces between the highest Iroquois and the Gil- 

 bert planes have been wave-swept by the subsiding waters, and 

 many patches of bared rocks are found at various levels, no 

 beach phenomena have been noted between the two planes. All 

 the high level shoreline features in our district are confidently 

 referred to the sea-level waters. 1 



. 1 Since this paper has been in type Prof. George H. Chadwick discovered 

 heavy beaches and deltas of Lake Iroquois in St Lawrence county, and also 

 extensive deltas inferior to the Iroquois plane and of uncertain relationship. 

 In August 1910 we examined these features and carried the study north- 

 eastward into Canada. 



The Iroquois plane is now definitely known at several points, the farthest 

 east being at Chateaugay with altitude 975 feet. On the international 

 boundary ait Covey hill the full-hight plane is not much above 1000 feet. 

 The head of Covey gulf, the outlet of the lower or Second Iroquois, is 

 about 980 feet. 



A recent survey on the Canadian side of the boundary gives us precise 

 altitudes for the sea-level beaches (Gilbert gulf), which have at Covey Hill 

 post office a fright of a least 523 feet. 



These altitudes are entirely consistent with the figures and facts relating 

 to the Iroquois and Gilbert gulf water planes given in this report. 



