The blocks of the capping limestone at Niagara Falls are to some extent fixed in size by the joint planes present in them, and as they fall to the bottom of the gorge, they promote or retard the further recession of the Falls according as they can or cannot be moved about by the churning currents beneath the cataract. Of the retarding effect there is an illustration in the accumulation of the blocks below the American and the intermediate Luna Falls (plate 23 A), which the weaker currents upon the American side find too heavy to handle. [Illustration: FIG. 380.—Ideal section to show the nature of the drilling process beneath the cataract.] [Illustration: FIG. 381.—Plan and section of the Niagara gorge, showing how in each section the depth is proportional to the width, except in the lowest section where subsequent river action of the normal type has modified the bed of the channel (plan after Taylor and section after Gilbert).] The Canadian Fall, with its much greater power, is an example of the promotion of recession through the churning about of the blocks at the base of the cataract. We have here to do with a churn drill which bores its way into the bottom of the gorge with increasing radius of rotary motion with each increase in volume of the falling water. Under this rotary churning the soft shales are torn out near the bottom and in succession the harder layers above until the capping is reached (Fig. 380). The conditions appear now to be such that the effective work is largely concentrated, as it usually has been, near the middle of the channel; and so the gorge recedes with a margin of the earlier river bed remaining as a terrace on either side and extending to the former river bank (Fig. 377). As must have been noted, one peculiarity of the operation of the churn drill beneath the cataract is that the depth of the gorge will bear a direct proportion to its width, and if the volume of water has varied during the process of recession, these changes in volume will be registered in the width and also in the depth of that section of the gorge which was drilled at the time—the cross section of the gorge at any place is proportional to the volume of the water falling in the cataract which produced it, modified, however, by the competency to handle the joint blocks of definite size (Fig. 381). [Illustration: FIG. 382.—Comparison of a sketch of the Canadian Fall made with the aid of a camera lucida in 1827 with a photograph taken from the same view point in 1895 (after Gilbert).]