POST-NIPISSING GKEAT LAKES. 467 



bly lies near the hinge line of the most recent uplifts, but the position of the line in that region 

 is not definitely known. The outlets of Lakes Superior and Ontario are well within the area of 

 the recent uplifts. 



Lake Michigan-Huron (regarded as one lake) has its outlet toward the south in the area 

 of horizontality, and for this reason the northern uplifts have not backed the waters up on its 

 southern shores. Since the end of the Nipissing Great Lakes the water level south of the hinge 

 line in this lake has been permanent, except for lowering due to wearing down of the outlet. 

 The recent uplifts merely raised the northern shores and caused them to emerge without pro- 

 ducing drowning at the south. During the early part of the present post-Nipissing lake stage 

 Lake Superior and Lake Michigan-Huron were united by a strait and had the same southern 

 outlet. About 50 feet of uplift occurred at Sault Ste. Marie while this relation lasted. It is 

 only during the last 20 feet of uplift at Sault Ste. Marie that Lake Superior has been an inde- 

 pendent lake and has had its outlet in the area of uplift north of the hinge line. The emergence 

 effects on the lower Nipigon are due to the latest movement, whether now progressing or only 

 of recent date. 



On theoretical grounds it seems certain that very slow drowning would favor the growth 

 of great barriers and spits like those mentioned, for it would greatly facilitate the erosion of all 

 exposed shores and enormously augment the supply of beach-making material. But so also 

 would a rise of lake level due to increased volume of discharge, like that which affected Lakes 

 Erie and Ontario at the beginning of the present lake stage, and this might be accompanied by no 

 warping or uplifting whatever. In this case there would be sudden drowning of small amount 

 (10 or 12 feet) and a permanent lake level thereafter. Reasons have already been given (p. 465) 

 for believing that Lake Erie was not appreciably affected by the post-Nipissing uplifts. The 

 last drowning that affected it was due to increased volume and rise of lake level following a 

 long low stage. The modern great bars of Lake Erie seem therefore to be due to relatively 

 long permanence of lake level following a sudden rise of small amount. 



The spits on the shore of Lake Ontario seem to indicate the same general conditions of 

 formation as those of Lake Erie, both in distribution and in strength of development (including 

 duration of time involved). It seems impossible to reconcile the great spits at Toronto and 

 Hamilton with an uplift of appreciable rate now in progress or with a very recent uplift of more 

 than 10 or 15 feet. It seems more probable that a large increase in the volume of discharge 

 without concurrent or subsequent uplifting of the land has been the chief factor. In this 

 connection it should be noted that the extent of the post-INTipissiiig uplift east of Lake Huron 

 and Georgian Bay has not yet been determined by observation. It seems almost certain, 

 however, that the isobases bend gradually to the east in southwestern Ontario so as to leave the 

 outlet of Lake Erie at or south of the hinge line, and it is a question whether in passing farther 

 eastward they do not curve still more, turning to courses somewhat north of east, so as to leave 

 the outlet of Lake Ontario likewise at or near the hinge line. In this case the outlet of Lake 

 Ontario may have been affected little if any by the post-Nipissing uplift and the spits of this 

 lake may also be due to a rise in lake level caused by increased volume of discharge. 



From the Niagara gorge it is learned that the whole of the post-Nipissing uplift, amounting 

 to about 104 feet at North Bay, Ontario, has taken place in the last 3,000 or 3,500 years. It 

 seems probable that the greater part of it occurred in the early part of this period and very 

 little in the last few hundred years, but the facts now available do not appear to afford a 

 satisfactory basis for determining whether uplift is now in progress and what its rate is if it is 

 going on. 



GAGE READINGS AND PROGRESSING UPLIFT. 



Some years ago Gilbert investigated the question of progressing uplift by collecting and 

 critically examining and comparing the records of changes of lake level at selected points where 

 gages have been kept by the United States Lake Survey. 1 The result showed a very small 



i Gilbert, G. K., Modification of the Great Lakes by earth movement: Nat. Geog. Mag., vol. 8, 1897, pp. 233-247. Also more fully in Recent 

 earth movement in the Great Lakes region: Eighteenth Ann. Eept. U. S. Geol. Survey, pt. 2, 1S97, pp. 595-647. 



