Abstract
The seven mile gorge below Niagara Falls has been dug by the tumbling waters undercutting the hard capping rocks making the cataract crest. Underneath are softer rock layers that yield and wear away in the powerful swirl of the plunge basin, thus undermining the harder top layers until great blocks fall from the overhanging ledge for want of support. These blocks, swept around by the mighty whirlpool of the falls, arm the water with effective cutting tools that dig the more swiftly into the rocks below. Thus the falls cut their way backwards, slowly receding upstream.