IV.—Valley Terraces

Extract

In the Journal of the Geological Society for November, 1866, page 463, Mr. Tylor gives a paper on “The interval of time which has passed between the formation of the upper and lower valley gravels.” He says, “Mr. Prestwich argues that, although the upper valley gravels are at a higher relative level than the lower, yet the higher series are always the older, and the lower the more modern; and we have thus the ordinary superpostion of new over old strata supposed to be reversed. This difficulty is considered by most geologists to have been surmounted by Mr. Prestwich's arguments, and his classification of the gravels has been generally accepted. My own opinion is that the evidence on which Mr. Prestwich's theory is based is insufficient.” If the gravel terraces were on one another as well as above one another, there would not only be a difficulty, but an impossibility to surmount. But the terraces are not on one another—that is, althought they are in level above one another, they are not vertically above one another. If in making a railroad cutting the sides were terraced, the upper terraces would be the first formed, and would consequently be “the older.” The lower terraces would be the last formed, and would consequently be “the more modern.” Mr. Tylor himself will allow that the upper part of a railroad cutting must be excavated before the lower part of it; and that if deposits were made during the excavation, the upper deposits would be the first made or “the older;” the lower deposits would be the last made or “the more modern.” The case is the same if, instead of the formation of a valley, and the excavators to be rain and rivers.