Volume 1

A manual of the geology of India / chiefly compiled from the observations of the Geological Survey.

Date:
1879-1887
Catalogue details

Licence: Public Domain Mark

Credit: A manual of the geology of India / chiefly compiled from the observations of the Geological Survey. Source: Wellcome Collection.

    493/554 (page 387)
    TAPTI AND PURNA ALLUVIAL PLAINS. 3S7 Their absence is not surprising, because land shells, for the most part, float when washed away, and are left on the surface, where they decom- pose instead of being preserved in alluvial deposits.1 Fluviatile origin of Narbada alluvium.—The examination of the molluscan remains in the Narbada clays and gravels completely dis- proves the idea of a marine origin, but it has been considered by some observers that the deposits are lacustrine.2 This view was principally based upon the uniform appearance of the clay and the absence of stratification. But this very uniformity and want of stratification are common characters of undoubted river deposits, and may be observed on the banks of most large streams, whilst the frequent deposition of pebble beds throughout the clays could not have taken place in the waters of a large lake. The bones, too, are isolated and broken, sometimes being even rolled, whereas, if deposited in a lake, different bones would in all probability be found together, because away from the margin there could be no current in the lake of sufficient force to transport bones divest- ed of flesh, and any mammalian remains deposited in the bottom of the lake must be derived from floating carcases or portions of carcases. Moreover, the Chelonia and fresh-water mollusca are all forms which inhabit either rivers or shallow marshes in river valleys, and it is impro- bable, if so great a change took place in the area, as would be involved in the replacement of immense lakes, or chains of lakes, by a river valley, that a greater difference would not be produced between the tor- toises and fresh-water shells formerly inhabiting the waters and those still living. On the other hand, the fact, that the alluvial formation occupies a rock hasin, shews that if the deposits were produced by a river, a considerable upheaval of land must have taken place to the westward. The river Narbada, moreover, is now no longer a depositing stream within the alluvial basin; on the contrary, it has cut its way through a considerable thickness of clays, and it must therefore have a greater fall than formerly. This, of course, may be due to the rocky edge of the basin having been cut through by the river, and by the channel of exit being consequently at a lower level. Tapti and Purna alluvial plains.—The alluvial plains of the Tapti valley require but brief notice.3 In their principal characters they resemble the Narbada plain, but the depth of the deposits is un- known, no deep borings having been made. As in the Narbada valley, the river now runs at a considerable depth below the alluvial plain’ 1 See Mem. ft. S. I., VI, p. (231). 2 Mem. G. S. I., II, p. 283. 3 For a few additional details see Mem. G S I VI, pp. (276), (286) ; and Wynne, Rec.