468 J. W. Spencer — Duration of Niagara Falls. 



ent level, 320 feet below the rapids above the falls. The com- 

 mencement of the work of this epoch was taken where the 

 canon suddenly became broad at the head of the whirlpool 

 rapids, a phenomena explained by the force of the river being 

 vertically diminished and latterly increased — the converse to 

 the conditions of those of the third episode. At first the rocks 



Fig. 10. — Section across the gorge at Johnson's ridge (ee, fig. 1) ; L. , level of 

 Lake Ontario; r, surface of river; b, original bank of river; bottom of $iver 80 

 feet below surface of the lake. 



in Johnson's ridge offered great resistance on account of the 

 increased thickness of limestones, nevertheless the lateral ero- 

 sion gained the ascendancy over the vertical. The section 

 through Johnson's ridge is 5,500 feet long, and with the laws 

 of erosion the time necessary for the falls to retreat through it 

 would be about 1,500 years — thus would end the first stage of 

 the last episode. The last stage is the modern, or that since 

 the cataract reached the Tonawanda basin south of Johnson's 



■:.''.. 

 



1^^- 







;?.^*!£w.« 



T77^ Goat Islan 



Drift * 





Nidsara 



-_-.-.--• .-]\ l~--- ■-,- •- - i_-_-i 



500 1O0O 

 *— ' — 1 ft 



Clinton 





i. Medina 













^fJ^W^^\ /f^I-Ii-i-Ax.^ 



Fig 11. — Section across gorge 1,000 feet north of the Horseshoe falls (cc, fig. 1 

 L, Lundy beach to the west; t, terrace with sandy face; ra, surface of river at 

 crest of falls ; rs, ditto below falls: A?-, ditto of American falls ; L. 0., level of 

 Lake Ontario. Bottom 80 feet below lake surface. 



ridge, whose rocky floor, generally speaking, is about 80-90 

 feet lower than that on the ridge (see fig. 8) ; yet the canon 

 just north of the ridge is only 250 feet wider than through 

 that barrier. The drift filling the basin offered but little 

 resistance to the recession of the falls and accordingly the rate 

 of retreat has been comparatively rapid along this section of 

 the river, which is 6,000 feet long. Consequently its age is 

 about 1,500 years. Thus the duration of the fourth epoch has 

 been 3,000 years. 



Age of Falls. — Allowing 1,000 yearsfor the duration of the 

 river before the advent of the falls, — for that its commence- 

 ment was not characterized by a cascade is shown by the ter- 

 races on the edge of the escarpment and at the deserted mouth 



