EVIDENCE FROM AEARGINAL BELT WEST OF NEW ENGLAND 311 

 ABANDONED LAKE STRANDS 



At first sight the old shorelines of the Great Lakes appear to negative 

 the hypothesis of marginal downwarping since the Wisconsin stage of 

 glaciation. Southwest of the Whittlesey hinge-line, Leverett and Taylor 

 find the abandoned strands to be almost perfectly horizontal. For ex- 

 ample, in Michigan alone their sections show horizontality for the Whit- 

 tlesey shore through a distance of 65 miles. Assuming their zero isobase 

 and assuming also that they have correctly identified the beaches, the 

 Jamieson hypothesis is decidedly not favored by their field observations. 

 These would point rather to the hypothesis of purely elastic deformation 

 if the rise to the northward is isostatic; or to the assumption of practi- 

 cally perfect fluidity for the subcrustal material, if the isostatic adjust- 

 ment took place through "plastic'^ deformation. 



Since both deductions are antecedently improbable, the search for evi- 

 dence of a marginal bulge south of the Great Lakes should not be given 

 up. According to Leverett and Taylor, the Whittlesey hinge-line is only 

 approximately placed on the map, and they note the uncertainty that it 

 does really represent the zero isobase. The older Maumee beaches in the 

 "area of horizontality" show some slope to the southwest, and all of that 

 slope may not be due to ice attraction, as held by the authors quoted. ^^ 

 If so, the zero isobase is somewhat south of the mapped Whittlesey hinge 

 line. The lake strands may thus lie outside the critical area where such 

 levels might give evidence of crustal downwarping. The delicacy of the 

 hypsometric problem is illustrated in Brogger's map of the analogous 

 isobases crossing the west coast of Denmark. Twenty-two kilometers 

 northeast of the zero isobase the uplift was only 2.5 meters and a sub- 

 sidence of equal amount is indicated at about the same distance southwest 

 of the zero isobase. If correct in principle, Brogger's map suggests that 

 the hinge area in America may be broad and characterized by but very 

 slight departures from true horizontality. Moreover, the strand-lines in 

 uplifted Ontario now show broad, almost perfectly level "treads," sepa- 

 rated by areas of tilted beaches. Is the "area of horizontality" adjoining 

 the Whittlesey hinge-line in a similar relation, the true isobase for zero 

 being southwest of this region of old strands? 



On the other hand, the strands of the late Wisconsin Lake Passaic in 

 New Jersey are rather strongly tilted in the direction demanded by 

 Jamieson's hypothesis. In a distance of not more than 25 miles the tilt 

 measures 67 feet and it is in a southerly direction. ^^ According to AVood- 



" F. Leverett and F. B. Taylor : Monograph 5.3, U. S. Geol. Survey, 1915, pp. 348, 

 377, plate xx. 



" R. D. Salisbury : Final Kept. State Geologist of New Jersey, vol. 5, 1902, p. 224. 



