Sedimentary Formations of Abnormal Thickness

Extract

For many years past the writer of the present note has devoted much thought to the problem of the origin of sedimentary series of enormous thickness, such as occur or are said to occur in many parts of the world. In one group of instances the answer is easy: that the great thickness is apparent and not real. Such for example are some types of delta deposit, where the material has been laid down on a submarine slope in a manner analogous to the formation of a railway embankment or a slag heap, by tipping trucks over the end. Here obviously the total attained by adding together the thicknesses of all the inclined layers is quite misleading. This effect may be seen in the foreset beds of almost any deltaic deposit on almost all scales, and its true nature is generally revealed by the occurrence of topset or bottomset, more or less horizontal beds above or below. This effect is very well seen on the south coast of Devon westwards from the mouth of the Exe, where for some miles the bright red New Red Sandstone strata, usually considered to be Permian, dip consistently eastwards at 25 or 30 degrees. The conditions of formation of these brilliantly red strata are obscure, but in one railway cutting at the eastern end an obvious horizontal topset bed proves clearly that the dip is deceptive, and that the real thickness at that point need not be much more than the small depth of the cutting.