A RECONSTRUCTION OF WATER PLANES 467 



THE ALGONQUIN BEACH 



Below the beaches of Lake Chicago, encircHng the south end of 

 Lake Michigan, is the Algonquin beach. It rises northward, in 

 the central and northern part of the basin, as shown by the map. 

 Fig. I. The identity of this beach, the highest shore line on Mackinac 

 Island, as the "Algonquin" beach' of Spencer was long ago recog- 

 nized by Taylor. On the map. Fig. i, can be seen the altitude of this 

 Algonquin beach at about thirty-five selected localities in the Huron 

 and Michigan basins. The warped attitude of the water plane 

 which passes through these points is indicated by the system of iso- 

 bases and the line of maximum inclination which runs perpendicular 

 to them. The data have been taken from several sources. The 

 measurement on the Garden peninsula (725') is one of many made by 

 Hobbs in 1907. The twenty or more remaining measurements around 

 Lake Michigan and the Straits of Mackinac were made by the present 

 writer, in company with F, B. Taylor, in 1907. On the west side 

 of Lake Huron the measurements were made by Frank Leverett, 

 A. C. Lane, W. M. Gregory, W. F. Cooper, and C. A. Davis. East 

 of Lake Huron the measurements are all Spencer's except the one at 

 Beaverton, which was recently made by Taylor. 



Recent investigations by Taylor in Ontario, supplementing earlier 

 studies, indicate that this beach marks a period of activity of two 

 outlets, one at Port Huron and one east of Kirkfield, Ontario, where 

 there was an overflow into the valley of the Trent River. This 

 Algonquin beach, of the "two-outlet" stage, seems to be the highest 

 beach common to the Huron and Michigan basins. This gives 

 reason to conclude that when the ice border south of the Straits of 

 Mackinac withdrew so as to let the waters of the Michigan basin 

 merge with those of the Huron basin, the "Trent outlet" was already 

 running. Had the lakes merged before the Trent pass was uncovered 

 and while the Port Huron outlet alone was active, then the plane of 

 that beach, adjusted to the Port Huron outlet, would have been tem- 

 porarily abandoned when the Trent pass was uncovered, and the 

 waters fell to a low level ; and the subsequent uplifts which raised the 



I J. W. Spencer, "Notes on the Origin and History of the Great Lakes of North 

 America" fabstract), Am. Assoc. Adv. Sci., Proc, Vol. XXXVII, pp. 197-99, 1889, 

 and later papers. 



