Geomorphological Forms Research Articles

River valley anomalies — One approach to the study of Fennoscandian bedrock relief

The intricate problem of major Fennoscandian landforms has been studied along the following lines: Structural control, erosion surfaces, climatic geomorphological forms, glacial erosion, and river valley anomalies. The last-mentioned method of approach was tried several decades ago and is almost forgotten. It will be discussed here. Of special interest are the river captures at the main water divide. Clear cases are few compared with the great difference in river length on both sides. River gaps form the main group of discussed cases; the most striking ones are in the Swedish Caledonides. Less well known are the water gaps in pre-Cambrian Sweden. Local tectonic movements cannot be excluded as an explanation in most cases, but are not necessarily the explanation. More probable as a general explanation are lost sedimentary rocks, notably Cambro-Silurian. This means a stable shield area, slowly being stripped of a sedimentary cover in relation to varying rock resistance and undulations of the crystalline bas...

Alluvial sedimentology of the Upper Pleistocene Hinuera formation, Hamilton Basin, New Zealand

Abstract The Upper Pleistocene HinueraFormation is an unconsolidated alluvial deposit, up to 90 m thick, underlying some 2000 km2 of the plains of the Hamilton Basin and the southern half of the Hauraki Lowland. The sediments have been studied in detail at 19 widely-spaced sections in Hamilton Basin. The most striking lithological features are their coarseness, acid volcanic composition, and abundant cross-stratification. Sediments are mainly gravelly or slightly gravelly sands, sandy gravels and silts; peat beds are developed locally. Gravel-sized material is dominated by fragments of rhyolitic breccia, rhyolite, pumice and ignimbrite; sand and silt fractions by volcanic quartz, oligoclase/andesine plagioclase, pumice, and glass shards. Heavy minerals are hypersthene, magnetite, hornblende, augite, epidote, and biotite. The minor amounts of clay minerals include halloysite, kaolinite, illite, and chlorite. Sedimentary structures are dominated by thick cosets of cross-stratified gravelly sands and sandy gravels composed mainly of lithologically heterogeneous crossstrata. On the basis of height, Cross-strata fall naturally into small <5 cm), medium (5–15 cm) and large (>15 cm) scale sets. Four types of cross-stratification (Rho, Nu, Epsilon, and Sigma) are recorded. Rho and Sigma varieties are new and consist of grouped, medium- and large-scale, lithologically heterogeneous sets with erosional bases which in the case of Rho cross-stratification are scoop-shaped and overlain by discordant cross-strata but for Sigma cross-stratification are planar and overlain by concordant cross-strata. Horizontally stratified and massive units are locally common, particularly in pumice silts and sands. 'Sedimentary structures in sands and gravels are the product of specific bed forms developed on longitudinal and transverse channel bars under both lower- and upper-flow regime current characteristics. Liquefaction produced a variety of post-depositional deformation structures in gravelly sands and silts. Grain size parameters are highly variable. 'Size frequency curves for sands and gravels portray a significant contact load population, a well-developed (up to 90%), exceptionally coarse and variably sorted saltation population, and a small, commonly coarse, suspension population. Complex grain size distributions result from mixing of up to four log-normally distributed size populations in polymodal sedinlents. Textural data indicate that the bulk .of the Hinuera Formation was deposited rapidly in an environment of fluctuating high turbulent energy. From composition, texture and sedimentary structures seven lithofacies are erected for the Hinuera Formation which correspDndto deposits formed in a variety of subenvironments in a braided river system of the ancestral Waikato River. The geomorphological form of the formation in Hamilton Basin is ,that of a large, very low angle alluvial fan, formed from the debouching of sediment through the narrow Maungatautiri Gorge out into the basin. Aggradation occurred in two phases, both initiated by vDlcanism in the Central Volcanic Region, and sustained by increased rates of erosion in the Upper 'Waikato Basin under a cold, seasonally wet climate.

Coastal and nearshore sedimentation: A comparison of clastic and carbonate deposition

Two coastal and nearshore areas of extensive Recent sedimentation are compared: the Trucial Coast of the Persian Gulf dominated by carbonate deposition, and the Dutch Wadden Sea of the North Sea dominated by elastic deposition. The paper stresses that comparable patterns of environmental conditions have produced similar geomorphological forms and analogous patterns of sediments which do, however, reflect the differences in sediment source and climate in the two areas. If the sequences are ultimately preserved, the Trucial Coast will produce a limestone-evaporite sequence and the Dutch Wadden Sea a sandstone-shale-coal sequence.

The Problem of Coastal Genesis, with Special Reference to the "Strandflat," the "Banks," or "Grounds," and "Deep Channels" of the Norwegian and Greenland Coasts

The so-called strandflat of the Norwegian coasts has aroused the interest of scientists since an early date. Later other forms, such as banks or grounds and deep channels, were discovered and posed additional problems. But no agreement has ever been reached on these problems in spite of numerous discussions. The formation and processes in the development of the strandflat and adjacent geomorphological forms cannot-according to the author-be separated from the history of the hinterland. Thus, to a major extent, the strandflat forms a part of a polycyclic piedmonttreppe. In the same way the bankflat is stated to be identical with one step of this geomorphological staircase. The banks and grounds off the coasts are explained as galacial-fluviatile-marine deposits of a very complex nature, intersected by longitudinal and transverse channels. These peculiar submarine forms are suggested as having been created by fluvial and glacial erosion and conserved to our day by a system of marine currents. Isostatic and eustatic movements are additional factors to be taken into consideration in the history of the building of the coast forms mentioned.

Estuarine Meanders in the Chesapeake Bay Area

M AORPHOLOGISTS have accumulated a wealth of observations and conclusions on the development of coasts and shore lines and on the sculpturing work of rivers above tide level, but the landform that owes its configuration to the combined effects of tidal action and river flow, the estuary, has been somewhat neglected-perhaps because its origin has seemed too fluviatile to coastal morphologists and too marine to those concerned with river work. This may explain why the particular geomorphological form discussed here has escaped scrutiny for so long a time, in spite of its prominence and wide occurrence. Past investigators' of meandering tidal streams were mostly concerned with the question whether rivers at tide level can or cannot meander actively; they did not study the form differences between river meanders above tide and meanders shaped with the participation of periodically reversing tidal currents. Numerous estuaries around Chesapeake Bay possess meanders of the latter origin. A check of topographic maps revealed their presence also in other parts of the Atlantic Coastal Plain of the United States, and even on coasts of other continents. These meanders are sufficiently different2 from other meander forms, and at the same time sufficiently frequent, to be classified as representatives of a new meander type, the estuarine meander. The present paper attempts to describe the major characteristics of this meander type.

Some Recent Geological Deposits in the Egyptian Nile Valley

Several years ago G. W. Murray I contributed a discussion to the Geographical Journal which m rked the first successful attempt o sketch the Upper Pleistocen and Holocene climatic history of Egypt in a truly modern fashion. The wealth of personal observations and intimate knowledge of the material described reflected Mr. Murray's long years of experience in Egypt, since 1941 as Director of the Topographi? cal Survey. During the winter of 1958 the present writer collected geological data in Middle and Upper Egypt which adds in some way to our present knowledge of Postglacial climates in that country. Although the Recent, or Holocene, deposits of Egypt form but a minute fraction of the surface area, their significance for the inhabited riverain zone is paramount. Climate and sedimentation must have played as important a role in modelling the geographical environment in ancient times as they do today. For example, it is not at all obvious that the precious Nile alluvium has been distributed and laid down at a uniform rate since the close of the Pleistocene, as is widely supposed. These Holocene deposits in the Nile Valley are of aeolian and fluviatile origin: sand dunes, nilotic mud and wadi deposits. The aeolian deposits.?Wind-borne sediments in the valley are more or less limited to the western margins, particularly between Gebel Deshasha (near Biba) and the monas? tery Deir el-Miharraq by el Qusiya, a stretch of over 80 miles. These sand fields are of considerable hindrance to cultivation. Their development is both complex and interesting, so that it is regrettable that, apart from reference to their existence, little attention has ever been paid to them. Firstly one can speak of an irregular line of marginal valley dunes lying exlusively upon Nile mud to a width of one half to two miles between Deshasha and a point about three miles south of Balansura (west of Abu Qurqas). After a small interruption similar dune fields occur between Tuna el Gebel (west of Mallawi) and Dashlut (west of Dairut), and again between Nazlet Bawit (west of Sanabu) and el-Miharraq. In contrast to the dunes upon the Pleistocene gravels west of the desert margin, these are to a fair extent fixed by vegetation deriving its moisture from the ground-water. Although the fields between Deshasha and Bal? ansura can only be classified with difficulty as generally transversal arrays running paral? lel to the border desert-alluvium, those between Tuna and Dashlut are developed as specific geomorphological forms. Three rows of 10 feet high transversal dunes run NNE.-SSW. while another line abuts the desert in the lee of the Pleistocene gravels. In the Meir area, between Sanabu and Qusiya, two immense longitudinal dunes of four miles in length are blown up in the lee of the steep limestone scarp running NNW.-SSE. All these dunes overlie nilotic sediments and were apparently deposited during the last few centuries. From behind the Black Hills due west of Beni Mazar a chain of NNW.-SSE. longitudinal dunes, generally averaging one or two miles in length, extends over 30 miles across the desert surface to the nummulitic headland a little south of Balansura.