Drift Areas Research Articles

Distribution and Aspects of the Ecology of the Genus Tabellaria Ehr. (Bacillariophyceae) in the Northcentral United States

The ecology and distribution of four taxa of the diatom genus Tabellar?a (Bacillariophyceae), recognized by Koppen in the N- central United States, were investigated. All four were abundant in the portion of the study area mantled with noncalcareous to weakly cal- careous surficial deposits. In addition, the distribution of the genus correlated with the dominant presettlement vegetation pattern in the study area. Tabellar?a fenestrata and T. floeculosa strain IV were, for the most part, restricted to waters located in noncalcareous to weakly calcareous surficial deposits. These waters tend to be very soft to soft and oligo- dystrophic. The distribution of T. flocculosa var. linearis extends from the noncalcareous to calcareous drift areas, occurring in some lakes which are classified as moderately hard-water to hard-water and meso- trophic to eutrophic. Tabellar?a flocculosa strain III was encountered in all types of waters described above and in addition was found in many more lakes located in areas with calcareous deposits and considered as very hard-water and eutrophic.

Notes on Middle Stone Age Sites in the Muden/Keates Drift Areas of Natal

Notes on Middle Stone Age Sites in the Muden/Keates Drift Areas of Natal

The Origin of Lake Basins

I HAVE been following with some interest the recent discussion in these columns concerning the power of ice to erode rock basins, since I have prepared for publication (presented at the December, 1893, meeting of the Geological Society of America) a paper describing some recent studies of my own upon this subject. After several years of study in the glacial belt of New England, having never found definite evidence of rock basins in lakes of large size, I came to the conclusion that the theory of rock basins had little value, particularly since, after having been before us for more than thirty years, so few instances have been proven. In my own case, and I believe also in others, the attempt had always been to trace a continuous rock line, and this I now believe to have been an entirely wrong method; for how many large lakes are there in which possible outlets may not be buried beneath drift areas?

On the Origin of Dry Chalk Valleys and of Coombe Rock

For many years the singular mass of angular flints and Chalk, known as the Brighton Elephant-bed, has been familiar to geologists. This deposit I had not seen till 1884, when I was instructed to examine, for the Geological Survey, the Pleistocene deposits of Sussex between the escarpment of the South Downs and the sea. Coming from eight years' work in strongly glaciated districts, I was at once struck by the appearance of the Elephant-bed—or, as it is called in the district, the “Coombe Rock.” It is a very different deposit from anything commonly seen in the Yorkshire or Lincolnshire Wolds, and different, though not so markedly different, from anything found in Norfolk. This occurrence in a non-glaciated district of a type of gravel unlike anything of ordinary occurrence in glaciated districts of similar configuration aroused my interest. After two years' study of the beds in the field, I venture to bring forward my views as to the mode of formation of Coombe Rock and as to the origin of dry Chalk Valleys—two subjects intimately connected. The configuration of the surface beneath the drift on the seaward side of the South Downs is identical with that found in the Chalk districts of Yorkshire and Lincolnshire. In each of these districts we have a dip-slope from the edge of the escarpment seaward. But this slope does not pass under the low-lying drift areas; it ends abruptly in a cliff, now much degraded, but still recognizable as a sea-cliff both by the marine

Additional Discoveries of High-level Marine Drifts in North Wales, with Remarks on Driftless Areas

Summary of Facts and Inferences In speculating on the origin of the high-level marine drifts described in this and in previous papers, the following facts and inferences ought to be duly taken into consideration:— 1. High-level marine drifts, in addition to directly local stones and stones from neighbouring areas, consist to a greater or less extent of far-travelled erratic stones, while in none of them are the stones entirely local. 2. The stones are generally much rounded, and that frequently over large areas. 3. All the great drift-areas contain a considerable, and some of them a large percentage of granite, especially Eskdale granite, which is at least partly owing to the extent to which the Eskdale area is covered with granite fragments and pebbles in positions favourable to removal by floating ice. 4. All the great drift areas (so far as yet known) are situated on or towards the outer slopes of mountain districts. 5. In the interior of mountain districts high-level drifts are either absent or limited to patches. 6. The drift-areas are more or less bounded above and below, as well as longitudinally, by areas in which the stones are angular or subangular. 7. None of them are situated further south than latitudes in which both land ice and floating ice may at one time have existed. 8. In all the areas there is a tendency to a knoll-shaped configuration, and in a greater or less number of instances a tendency in the knolls to occupy perched positions. 9. The gravel and sand generally contains very few large boulders; but the latter are common in and on the surface of the clay, which generally overlies, but in many instances graduates (on the same horizon) into gravel and sand. 10. The shells found in the drift are almost universally fragmentary, and that often, if not generally, in proportion to the roundness of the associated stones. They are likewise often confined to particular spots, as if elsewhere they had never been present, or had been destroyed by the stranding of floating ice, as is the case on many Arctic sea-shores at the present day. 11. The idea of shell-fragments having been pushed up hill along with portions of existing sea-beds is opposed by so many facts as to render it altogether untenable. 12. The extent to which the form of the ground has been preserved since the close of the great submergence can only be explained by supposing that either the time which has elapsed since that event, or the rate of subaërial denudation during that time, has been overestimated; in other words, the denudation must have been slow in proportion to the length of time in order to account for the little-altered surface-configuration of the drift-areas described in this and in my former paper.