. Bulletin of the Geological Society of America. Geology; Geology -- United States. THE ORIGIN OF CROSS-BEDDING. 199 frontal lobe of a sand plain near Wakefield, both in eastern Massachusetts. It is manifest that both were built by a stream moving to the right in the figure. This corresponds to the direction from head to front of the plains, and indicates that the back-sets were built by an ascending stream, rising. Figure 4.— Cross-bedding at the Front of a Sand Plain. from beneath the ice to the top of the delta plain, while the fore-sets were built by a descending stream, flowing from the p
. Bulletin of the Geological Society of America. Geology; Geology -- United States. THE ORIGIN OF CROSS-BEDDING. 199 frontal lobe of a sand plain near Wakefield, both in eastern Massachusetts. It is manifest that both were built by a stream moving to the right in the figure. This corresponds to the direction from head to front of the plains, and indicates that the back-sets were built by an ascending stream, rising. Figure 4.— Cross-bedding at the Front of a Sand Plain. from beneath the ice to the top of the delta plain, while the fore-sets were built by a descending stream, flowing from the plain into the water at its front. Ratio of Sand-plain Groivth to Ice Melting.—The ratio of fore-set and back- set beds is of interest, for, as already stated, it indicates the ratio of the for- ward growth of the delta to the backward melting of the ice. The sections thus far examined do not furnish final numerical results; but enough has been seen to make it clear that the fore-sets are from ten to forty or fifty times as extensive as the back-sets, and from this it appears that the melting of the ice was slow compared to the growth of the delta plain. Origin of Depressions in Sand Plains.—This conclusion is of value in ex- plaining the pits, kettles, and irregular depressions that frequently interrupt the otherwise level surface of the plain. The theory has long been current that these pits were the sites of isolated blocks of ice, around which the sands of the plain were deposited ; but it has also been currently objected to such an explanation that it involved an improbable and unproved rapidity of sand-plain growth. The conclusion just gained from the ratio of the fore- sets to the back-sets overcomes this objection. No satisfactory section of the slopes of a pit has, however, yet been found to give more direct evidence on this Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - color
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