In order to obtain a sure means for measuring the recession of the Falls, Professor James Hall made a survey of the Horseshoe Falls in 1842, under the authority of the New York Geological Survey. This survey plotted the position of the crest of the Falls, and established monuments at the points at which the angles were taken; thus leaving lasting marks of reference to which any future survey might be referred. In 1886, Professor Woodward of the United States Geological Survey, by reference to the markings left by Hall, found the rate of recession for the period to be about five feet per annum. It would, however, be necessary to extend these observations over a long period of time, since certain periods are marked by large falls of rock. Sometimes the centre of the Falls recedes very rapidly, while at other times the centre is almost stationary and the sides show the greater action. One of the most recent calculations of the age of the Falls was made by J. W. Spencer. Having made a thorough study of the history of the river revealed in its markings, and also of the Lakes, making allowance for all the variable factors, he calculated the duration of each epoch separately; and found the age of the river to be about 32,000 years. This result is about the same as that obtained from those based upon the relative elevations of different parts of the old deserted shore lines; and another based upon the rate of the rising of the land in the Niagara district. [Illustration: Cave of the Winds in Winter.] The many variable factors entering into the calculations so far discussed, have led to an earnest search for some means of determining the age of the river, which does not involve so many indeterminate and unknown quantities. This means of calculation, and one which seems to be much more free from unknown factors, seems to have been hit upon by Professor George Frederick Wright, whose calculations are based upon the rate of enlargement of the mouth of the river at the Niagara escarpment, where the Falls first began their existence. The cliffs at the mouth of the Gorge, as is the case with the newer portions of the river and indeed is characteristic of all canyons when first formed, were undoubtedly almost perpendicular when they were first cut by the rushing waters of the Niagara River. The mouth of the Gorge at Lewiston is of course the oldest part of the river; and if it were possible to measure the age of this part, this would surely give the date of the birth of Niagara. Immediately upon the formation of the Falls at Lewiston, the waters began the cutting of the Gorge; and immediately upon the formation of a gorge there was set to work upon its walls the disintegrating agencies of the atmosphere, free from indeterminate variables, tending to pull down the cliffs upon each side of the stream which jealously walled it in.