Investigations on a piedmont drift deposit in the foot-hills of the Eastern Himalayas and its glacial and periglacial significance

Abstract

The paper embodies the important results of investigations carried on by the author on a piedmont deposit extending for about 40 miles from east to west and for about 10-12 miles from the Sub-Himalayan zone (Siwaliks) in the north to the North Bengal Plains in the south. The drift beds more or less overlie the northward dipping, partly denuded strata of Siwaliks (Pliocene -Pleistocene), and attain a maximum vertical thickness of 400-500 ft. at the outlet of the Himalayan river Jaldhaka, having its origin in the glaciated tracts of south-eastern Sikkim. The thickness of the deposits decreases towards the plain to which it merges, though at one place it forms an abrupt cliff of about 250 ft. The surface of tlie drift bed is hummocky, and shows a chain of hillocks of drumlinoid form and arranged in an arcuate fashion. In all its surface expression it gives a picture of pseudomoraine or bouldery topography typical of gravel fans at the base of the high Himalayas. From a series of geomorphic investigations the author has corne to the conclusion that the drift beds owe their origin neither to marine action on a slowly rising land as conceived by Godwin-Austen in 1866 nor to subaerial recent deposition beloging to the Siwaliks group as hinted by Mallet (1874). It has been concluded by the author that the enormous depositions filling up the Jaldhaka and other Himalayan valleys represent a vigorous glacio-fluvial filling aided by solifluction etc. during the Pleistocene glaciation. They are cut by 3 systems of terraces corresponding to 3 glacial periods and that the extensive pseudomorainic deposits in the foot-bills and at the outlet of rivers belong to the ,,Boulder Conglomerate" group of the deposits in Western Himalayas. The very characteristic facetted, faintly striated and a high proportion of ,,flat-iron shaped" boulders traced by the author testify to the origin of the ”Boulder Conglomerates" as a glaciofluvial outwash. All these corroborate the findings and conclusions of De Terra in the Kashmir Himalayas in 1936. The present work traces for the first time the effect of Pleistocene glaciation on the lower hills and of the glacigenic character of ,,Boulder Conglomerate" beds in the Eastern Himalayas.