Flow Till Research Articles

Facies of supraglacial sedimentation on Icelandic and Alpine temperate glaciers

Supraglacial debris transported by temperate valley glaciers is classified as supraglacial morainic till, distinct from lodgement till, and melt-out and flow till species on polar glaciers. In Iceland and the Alps, where annual discharges of supraglacial morainic till vary from 200–2000 m3 (compared with a maximum discharge of 26 000 m3 of lodgement till), till is deposited as three facies.Fades 1 occurs where supraglacial morainic till slows the rate of ice melt such that till is slowly superimposed on the subglacial surface in the form of stagnation or disintegration topography. The rate of deposition shows an initial slowing phase in response to (1) soil formation and (2) melt-out of englacial debris, followed by accelerated deposition accompanying the formation of thaw lakes. Continued mass movement of till on lake margins (backwasting) is more effective in destroying the ice-core than either top or bottom melt and prohibits accumulation of a distinct melt-out sediment. This situation can be contrasted with the style of sedimentation at polar glaciers. Where the depositing glacier is inactive and uncontrolled an unlineated till surface, typical of stagnation or disintegration topography, develops. Significantly, in terms of Pleistocene reconstructions, tracts of stagnation topography are being constructed at the margins of certain active Icelandic glaciers by sequential stagnation of a marginal rim of ice as the active glacier retreats up-valley.Fades 2 occurs where the till cover is too thin or too coarse and ice melt is unretarded and supraglacial morainic till is deposited as a dispersed bouldery veneer by dumping during which gravity sorting occurs. Dump moraine ridges frequently show internal bedding being ice-contact screes at time of formation but are not ice-cored. An active glacier produces a controlled distribution of landforms resulting in a lineated till surface.Facies 3 refers to those stratigraphic sequences where irregular or lensate till horizons alternate repeatedly with ice-contact outwash. The stratigraphic sequence as a whole is defined here as a supraglacial morainic till complex. Well-sorted, clast-supported outwash sediments occur within complexes deposited at inactive low-angled ice margins. At active steep-fronted glaciers, where areas of the till-covered ice front are only episodically scoured, distinct flood-deposited matrix-supported outwash units are found. Their subaerial formation runs counter to recent published interpretation of sediments in certain Pleistocene eskers.

Crystalline manganese oxides in till, from Bridge of Don, Aberdeen

Synopsis Highly crystalline manganiferous blocks consisting mainly of manganite and pyrolusite occur in a supraglacial flow till. Field evidence indicates that mineralization, which is of high chemical purity and notably free of iron, occurred in situ. The segregation of manganese from iron probably occurred as a result of selective dissolution of manganese in waterlogged organic-rich soil or sediment. Manganese and associated barium may have been transported by groundwater to its present location largely in the form of colloidal hydrous manganese dioxide. Initial deposition appears to have been localized by impersistent clay bands, with further deposition being catalysed by previously deposited manganese oxides. Local topography and depth of the clay bands relative to a fluctuating water table were probably critical in localizing the mineralization.

Ice-marginal sedimentary, glacitectonic, and morphologic features of Pleistocene drift: An example from Newfoundland

A Pleistocene drift sequence in hummocky terrain along part of the southern Avalon Peninsula of Newfoundland is interpreted to comprise complexly interrelated lodgement till, melt-out till, flow till, supraglacial and proglacial outwash, and supraglacial rhythmites. The gray and tan melt-out tills are stacked in imbricate fashion, giving rise to exceptionally thick stratigraphic sections. Contacts between melt-out tills are interpreted as remnants of shear planes because they are sharp, they dip in the up-ice direction, and they converge toward valley margins. Overlying flow tills interdigitate with supraglacial outwash. The drift sequence was deposited during a single episode of glaciation, rather than by repeated glacier advance, as previously proposed. It is the product of thrusting of englacial debris along ice-marginal shear planes, subsequent melting-out of englacial debris, and formation of supraglacial flow till and outwash. Preservation of this sequence probably is due to high content of englacial debris within the Wisconsinan ice. The sedimentary, glacitectonic, and morphologic features of this sequence are similar to those found at the margins of certain Arctic glaciers of subpolar thermal regime which have recently been the subject of Pleistocene glacial sedimentation models for west-central Canada and Great Britain. Recognition of these distinct elements indicates wisconsinan glacier lobes were of the cold Arctic type in southeastern Newfoundland. Alternative explanations for this sequence, such as deposition by glaciers of temperate thermal regime or by surging glaciers, are discounted. Because the features described here are complex and difficult to recognize, they may be more widespread in Pleistocene drift than has previously been interpreted.

Supposed area‐wasting of the Weichselian ice sheet in Denmark

The deglaciation at the end of the Weichselian in the Danish area has previously been considered to occur as a frontal wastage. Since the glacier ice was assumed to be debris‐free, the wasting should be characterized by outwash plains and successions of end‐moraines. The almost complete lack of sandur plains in the eastern part of the area and indications from recent investigations of widespread occurrence of flow till justify a re‐evaluation of the mentioned deglaciation model.Two morphological features have a general occurrence: the plains and the ‘tunnel’ valleys. The plains appear stepwise in the landscapes, and are frequently limited by steep slopes. Topmost is a subcircular kame‐like hill. Sedimentologically, the plains mainly consist of melt water deposits, and the scattered occurrences of till are interpreted as flow till. The plains continue from the open landscape into the ‘tunnel’ valleys where they appear as terraces.These features are considered to have been formed during the deglaciation. The almost horizontal surface of the ice sheet over large areas caused a sensitivity to changes in the climate. The wasting of the ice may therefore be expected to affect large areas almost simultaneously. On the assumption that the ice contained debris, an increasing amount of clastic matter was released on the ice surface. This material was concentrated in the depressions. If such a depression perforated the ice, the content of sediments settled on the substratum and a plain was established. During continued wasting the thickness of the ice decreased and the depressions were enlarged. They assumed the character of sandur plains. As still larger areas of these supraglacial sandurs rested on the basement the successive lower situated plains were formed. The latest ice was preserved where the ‘tunnel’ valleys are situated to‐day.

Lodgement till and flow till: a discussion

A comment by J. Kriiger on I. Marcussen's paper ‘Distinguishing between lodgement till and flow till in Weichselian deposits’(Boreas 4, pp. 113–123) is followed by a reply from I. Marcussen.

The influence of genetic processes on some geotechnical properties of glacial tills

Summary The principle characteristics of debris in transport by glaciers are described and the way in which this material is subsequently deposited as till. Three principal types of till are described, lodgement till, melt-out till and flow till, which can be related to typical sediment/landform models and which can form a good basis for site investigation planning. A proglacial/sub-glacial sediment association and land system is defined, in which lodgement till and proglacial outwash are the principal sediments, and in which drumlins, fluted moraines and push moraines, and kettle holes and eskers are the principal landforms. A supraglacial sediment association and land system is also defined, in which a suite of supraglacial sediments and landforms are superimposed on the subglacial/proglacial land system. The sediments comprise flow till, melt-out till and supraglacial outwash sediments, and the landforms largely comprise kame moraines. A third land system, the glaciated valley land system comprises chiefly medial and lateral moraines, in which the till is derived supraglacially from flanking valley sides, and kame terraces. Till genesis influences the geotechnical properties of tills through its control on several important characteristics: grain-size distribution and its spatial variation; the stress history of the till; the nature of the sequence in which the till lies, e.g. the presence of stratified horizons; and jointing in the till. Post-depositional processes which change these characteristics are also important. The processes considered are: post-depositional wetting and drying; freezing and thawing; remoulding; and downward percolation of fines.

Distinguishing between lodgement till and flow till in Weichselian deposits

Lodgement till and flow till are depositcd in quite different environments. In Weichselian depmits, they can be distinguishcd by the setting of the sediments, the fabric, geotechnical characters, and the morphological appearance. These indications support each other. Of the outlined methods, the morphological is the most convenient in practice. Since flow till normally exists together with meltwater sediments it is suggested that systematic morphological mapping will be time‐saving and most profitable for the prospecting of economically important clay, sand, and gravel deposits.

Glacial retreat sedimentation in the Smalfjord Formation, Late Precambrian, North Norway

ABSTRACTTillites, conglomerates and sandstones occurring in the basal part of the Smalfjord Formation along the Varangerfjord, East Finnmark, North Norway are believed to have formed during the retreat of a glacier.At Kvalnes, on the south side of the fjord, the following sequence, up to 20 m thick, is found: (1) massive monomict tillite interpreted as a subglacial till, (2) massive polymict tillite with lenticular intercalations of stratified sandstone and tillite, interpreted as supraglacial/proglacial drift, (3) polymict conglomerate interstratified with laminated sandstones, interpreted as braided stream deposits. The last named interfingers laterally and is overlain by marine sandstones.At Bigganjargga, near the head of the fjord, a lens of tillite about 3 m thick rests on a striated pavement and is overlain by sandstones and shales. Part of the tillite, containing irregular patches of slightly winnowed tillite, is interpreted as a melt‐out till, while a marginal part consisting of inclined tillite beds is interpreted as a series of flow till deposits. The lens is believed to be an oblique section through what was originally an ice‐cored moraine ridge. During a subsequent transgression, the moraine was partially eroded, a lag conglomerate was formed, and overlying marine sediments were deposited.Bedded flow tills formed in a supraglacial/proglacial environment may be preserved where the extent of current reworking is very low (such as an isolated end moraine). Stratified conglomerate and sandstone, intimately intercalated with tillite, is to be expected at a glacier margin where glacial meltwater is locally and occasionally abundant, and glacier ablation permits downslope flowage of mobilized supraglacial fluid till.

Studies on flow till in Denmark

Profiles with till and stratified meltwater sediments have been examined. Structural and morphological conditions indicate a deposition of till in the same environment as the stratified sediments. The till is interpreted as flow till which originated from considerable amounts of supraglacial material.

Modern Arctic glaciers as depositional models for former ice sheets

Sedimentary sequences currently forming at the margins of Spitsbergen glaciers are identical in thickness and detail to many Pleistocene and pre-Pleistocene glacigenic sequences. The transport of considerable volumes of englacial debris leads directly to the predominance of supraglacial till deposition, giving hummocky till surfaces and till plains. The association of supraglacial outwash with flow till produces tripartite till/outwash/till sequences, and multitill sequences, which are the result of a single glacier retreat phase. Complex tectonic structures, often with systematic regional trends are described, which are not the result of ice pushing but of downslope flow and collapse of supraglacial sequences above melting ice. New classifications are suggested for ice-contact stratified deposits and till, both of which depend upon position of deposition, supraglacial, englacial or subglacial It is suggested that existing models for the interpretation of ancient tills and the sequences in which they lie are often too simple and lead to erroneous stratigraphic and palaeogeographic conclusions. Till is too often interpreted solely as lodgement till, and it is suggested that many Pleistocene and earlier sequences, currently thought of as products of repeated glacier advance and readvance, may be perfectly normal products of a single retreat phase by a glacier with a thick englacial debris load. Ways of reconstructing the structural character of ancient ice margins are presented and it is also suggested that the thermal regimes of past ice sheets can be reconstructed from the nature of their deposits. The last Pleistocene ice sheet in Britain is thought to have been composed, at its maximum extent, of cold ice in the marginal zone and temperate ice in the internal zone.

NEW IRRIGATING VALVE WITH AUTOMATIC CUT-OFF AND REMOVABLE TIPS

To obviate the need of assistance to the patient by a nurse or a physician during nasal irrigations, I have devised a simple irrigating valve with an automatic cut-off, which is controlled by the patient. The photographs illustrate the device. In the figure, A shows the position of the plunger at rest, automatically cutting off the flow. B shows the plunger pressed in by the patient's thumb, permitting flow till released. C shows the nasal tip removed from the threaded end for sterilization. Another tip of any shape can easily be mounted. Both the automatic cut-off valve and appropriate nasal, aural, vaginal and rectal tips are made of hard rubber; the thread fits all these tips. This apparatus has certain distinct advantages: First, it is not attacked by antiseptics or disinfectants. Second, the patient, even a child, has complete control of both the duration and the

The Late Eruption of Vesuvius

OUR visit to the crater of Vesuvius on January 11, 1884, was a most interesting one. In my former letter I gave the rough details of this new eruption as well as could be ascertained from the base of the cone. The lava that issued on Tuesday night continued to flow till Wednesday evening, but seemed to have arrested its progress about 10 o'clock that night, when I was in the Atrio del Cavallo. This stream proved to have welled out at the base of the little cone of eruption and to have flowed across the solid lava plain in the crater of 1872, and then to have poured down the north-north-west slope of the cone till it reached the Atrio, across which it extended but little. Within the crater ot 1872 we have a somewhat convex plain of lava, which is continuous with, or, more properly, overlaps, the crater edges, except for a short distance on the south-south-west side. The northeast part of this is covered by the remnants of the crater of January, 1882. Within this were a series of crater rings that have since filled up to a certain extent the cavity of 1882. For some time the vent has travelled south, so that the present cone of eruption overlaps the crater ring of Januiry, 1882, on its south side, whereas there is a deep crescentic fossa between the present cone and the north crater ring of two years since. The vent was giving forth great volumes of vapour, and there was an almost continuous fountain of fragments of molten lava, which often attained the height of one or two hundred yards. As a consequence much filamentous lava, often as fine as cotton, was raining around the crater, and;as we sat there eating our lunch, it was so covered with these rock fragments, that it required a long climb on foot to make such a gritty meal palatable. The ejectamenta are composed solely of lava in detached pieces, ejected in a plastic state with a few bombs, consisting ot older solid lava fragments partially fused and rounded on the surface, which is varnished irregularly by the fluid magmi that enveloped them. This indicates that the lava is very near the top of the chimney, which must be full, as it has been for some time. Photography was no easy matter amidst this fiery bombardment, for such was the abundance of the ejectamenta that we could see how rapidly the cone of the eruption was growing. I made a rough calculation of the quantity of new material expelled, and 1 think six cartloads in four seconds as quite a fair estimate. The lava that had flowed was solid and cold enough to allow my dog to cross it with ease, though through a few cracks it was seen to be still incandescent, and a green staff thrust in immediately blazed. The lava that was flowing in the direction of Pompeii is still doing so in one or two points, apparently at the same rate and place as two weeks since.