CHARACTERS OF THE GORGE. 61 
‘ 
Horseshoe fall down to the upper side of Wintergreen flat. The map,* 
figure 1, shows the varying width of the gorge, and this, taken in connec- 
tion with varying depth, constitutes the main reliance for interpretation. 
DESCRIPTION OF GORGE CHARACTERS 
GEOLOGICAL CONDITION: 
The geological conditions at Niagara have already been so fully de- © 
seribed by Hall} and Spencer { that it seems unnecessary to repeat the 
description here. The strata as a whole lie ina position almost horizon- 
tal; so nearly so in the part of the gorge here considered that the very 
gentle southward dip may be neglected. The most important circum- 
stance, and the one which gives the falls their peculiar character, is the 
arrangement of the strata—a thick, hard layer above with a thick soft 
layer below. ‘This arrangement keeps the falls constantly vertical with 
a deep hole at their base. The recession of the falls and hence the mak- 
ing of the gorge are accomplished mainly by the cutting away of the soft 
beds in the bottom and sides of the hole. In this way the thick, hard 
bed of limestone above is undermined and falls piece by piece into the 
hole below, where the great blocks are spun round and round by the 
powerful currents and slowly grind away the softer shale. 
UPPER GREAT GORGE 
Beginning at the Horseshoe fall, let us note the chief characters of the 
gorge as we go down the river.§ Opposite the extreme west end of Goat 
island and just in front of the Horseshoe fall the top width of the gorge 
is about 1,250 feet. Thence it gradually widens to a point opposite the 
center of the American fall, where it is about 1,700 feet wide. A little 
* The accompanying map, figure 1, was reduced from a tracing of the “‘ Niagara Falls” sheet of 
the U. S$. Lake Survey (1875). The Survey chart has been followed closely, except for the vicinity 
of the Whirlpool basin, where the later survey of Mr G. K. Gilbert has been followed (‘‘ Niagara 
Falls and their History.” National Geographic Monographs, 1895, page 231). The heavy lines 
mark the upper edges of the cliffs on the two sides of the gorge ; the lighter lines the present water 
margin of the river, and the dotted lines in the vicinity of the Whirlpool the bluff edges of the 
eroded drift-filling of the old Saint Davids gorge. Hachures indicate rapids or turbulent water. 
The arrows in the Whirlpool and the smaller basin next aboye show return or backward currents. 
The places where critical events of gorge history occurred are known with only approximate acecu- 
racy, say within a few rods either way from the transverse lines by which they are represented on 
the map. 
{James Hall: ‘‘ Niagara falls, its past, present, and prospective condition.” Nat. Hist. of New 
York, Geology, part iv, chap. xx, 18438, p. 386. 
{J. W. Spencer: ‘“*The duration of Niagara falls.” Am. Jour. Sci., iii, vol. xlviii, Dee. 1894, pp. 
457-458. 
2 The Niagara chart of the U. S. Lake Survey is taken as the basis for all measurements of width 
and length. While it may not be entirely free from errors, as has been found to be the case for 
the topography at the Whirlpool on the Canadian side, it is, nevertheless, no doubt, substantially 
accurate and is the best authority at hand. 
