Drumlins and subglacial meltwater floods
|Since 1983, several investigators have developed a theory of drumlin
formation by catastrophic flooding due to the release of meltwater that
is believed to have accumulated beneath melting ice sheets. The proposed
catastrophic sheet floods, as wide as the drumlin fields, formed the drumlins
and related streamlined landforms, such as flutings, over wide areas. So-called
rogen moraine, consisting of transverse ridges of drift, often found associated
with drumlins, is reinterpreted in the meltwater flood hypothesis as possible
giant current ripples.
Existing drumlin fields reveal the extent of the areas affected by the proposed catastrophic floods. The Livingstone Lake drumlin field in Saskatchewan, where some very striking examples of streamlining occur, was the type region for the development of the new theory [Shaw et al., 1984]. The sheet flood flows proposed for this area by Shaw et al were about 150 km wide, and the total volume of the meltwater reservior was estimated as about 84,000 km3.
The subglacial flood hypothesis for drumlin origin has been applied to drumlins in New York, southern Ontario, the northeast shore of Georgian Bay in Ontario, and to the interpretation of hummocky terrain in southern Alberta, and other areas, by various workers. Shaw suggested as many as 10 meltwater discharges, similar to those which caused the Livingstone Lake drumlins, would be needed to account for drumlin fields in other areas [Shoemaker, 1995, p. 3].
The subglacial meltwater hypothesis of Shaw et al has several parallels to an interpretation of drumlins as streamlined landforms caused by the currents of flood waters generated by uplift, presented in the 1979 article Drumlins and Diluvial Currents by Cox. Although developed independently, Shaw's approach resembles that outlined by Cox in several ways. Cox published a photo with the above mentioned article showing drumlins in the Livingstone Lake area of northwestern Saskatchewan. These were some of the same ones that were the basis for Shaw's theory. Cox and Shaw both compared streamlined landforms produced in the Lake Missoula floods with typical drumlins, and followed the approach of the pioneering work of Sir James Hall in 1812, on the crag-and-tails of the region of Edinburgh, Scotland. Hall's interpretation invoked tidal waves from the sea, said to be harmonious with the Theory of the Earth proposed by his friend James Hutton.
Both Cox and Shaw related drumlins to transverse ripple-like features
that occur in association with drumlins. Cox, in
an appendix to his 1979 article, interpreted bedrock ridges on the Bruce
Peninsula in Ontario, northeast or upcurrent from the Sky Lake drumlin
field, as giant current ripples of bedrock, formed by transverse vortices
while the drumlins were being formed by longitudinal ones. Shaw and coworkers
have identified transverse rogen moraine ridges associated with drumlins
as features caused by the currents in the same subglacial environment as
the drumlins [Shaw, 1994, Munro
and Shaw, 1997].
The subglacial meltwater hypothesis proposed by Shaw invokes several catastrophic meltwater outbursts. The explanation outlined by Cox involves only one period of catastrophic erosion in which drumlins and a variety of other landforms were formed; the catastrophic currents were caused by tectonic adjustments associated with the retreat of the flood waters from the continents. Drumlins, giant current ripples, the Niagara Escarpment and its re-entrant valleys, as well as the deep lake basin of Georgian Bay, were all formed in these catastropic conditions.
Observations reported by Kor and others, who found evidence of catastrophic currents which eroded and streamlined granite bedrock on the northeast shore of Georgian Bay [Kor, Shaw and Sharpe, 1991], and additional evidence presented by Brennand and others [Brennand, Shaw, & Sharpe, 1996] for the regional scale of the former catastrophic currents, complement and support interpretations by Cox, that currents with southwesterly orientation, generated by uplift centered in the highlands of the Canadian Shield northeast of the area, streamlined and scoured the Bruce Peninsula. These crustal movements may have been associated with subsidence of the Michigan Basin. The southwest currents were probably effective in scouring long parallel grooves in the bedrock on the Lake Huron shore of the Bruce Peninsula at several locations. Similar grooves in bedrock are present at South Baymouth on Manitoulan Island. Their surfaces are striated and also pockmarked with concretionary cavities, resembling small partial potholes a few cm in diameter, indicating their erosion occurred as the rock was being lithified, aligned near the eroded surface. Subsequent currents with a southerly flow were involved in the excavation of Georgian Bay, as indicated by the patterns of drumlin orientation.
The following paragraphs list some of the outstanding questions and problems with the subglacial meltwater sheet flood hypothesis for drumlin origin:
How did melting occur under the ice?
How could the former ice sheets have melted from beneath? Most glaciers melt from above, and the energy source is sunlight and atmospheric heat. If it is claimed the heat came from the ground, this would be an argument against the plausibility of the growth of the hypothetical ice sheets in areas where they are said to have been. There is little evidence of volcanism in areas of the drift, so the source of subglacial heat is a problem. Alternatives to melting beneath the ice have been sought, such as meltwater originating at the surface of the ice, or in proglacial lakes [Shoemaker 1995].
Channeling of discharge
In the proposed subglacial meltwater floods, once a catastrophic release of pressurized meltwaters began, by failure of the ice lid on the subglacial reservoir, the currents of meltwater beneath the ice sheet would probably tend to be channeled, and initial conduits would probably become enlarged, causing a dendritic pattern of drainage. This mechanism is thought to have caused the erosion of tunnel channels in bedrock [Brennand & Shaw, 1993, Shaw, 1994], but where bedock channels and canyons are present, it seems unlikely that sheet floods would also occur. Capture of the majority of the flow by the initial conduits and enlargement of these conduits by erosion would limit the development of sheet floods over vast areas, as needed for the explanation of the drumlins by outburst of meltwaters from subglacial reservoirs.
Convergent vs divergent flow patterns
Patterns of flow of the drumlins would probably tend to converge towards the region of failure of the confined reservoir in the subglacial meltwater outburst hypothesis, which is contrary to observations of the patterns of flow typical of the drumlins. These patterns usually indicate a diverging flow and fan-like flow patterns.
Shifts in the flow orientation
In confined subglacial meltwater flow, the direction of currents would be strongly influenced by the shape of the cavities. Thus for pressurized flow, there would be some erosion of the overlying ice, where flow was rapid. The orientation of the flow would be determined by pressure. The fluid pressure everywhere within the system during the meltwater outburst would tend to be the same. A consequence of this is that it would be unlikely for the orientation of the general trend of the flow to change, as flow orientation would be determined entirely by the local pressure gradient.
Flow patterns from out of the sea
The orientation of the drumlins of some regions, notably northern Ireland, shows the direction of the former flow of the streamlining medium was from out of the sea. So how could there have been a catastrophic flow of meltwater from beneath hypothetical ice sheets from the sea onto higher ground? Why would the ice not have simply floated away? The orientation of drumlins and crag-and-tail features near Edinburgh, indicating flow patterns from the sea, led Sir James Hall to invoke tidal waves generated by earthquakes for their interpretation.
Limited size of vortices in the subglacial environment
In the proposed subglacial meltwater floods the estimated depth in most regions is about 10 m, but for the development of longitudinal vortices capable of forming the larger drumlins, the dimensions of vortices would probably have to be much larger, with diameters comparable to the height of the drumlins. This suggests the depth of flow must have been much greater than seems plausible for the subglacial meltwater floods. Rogen moraine ridges are typically spaced at about 300 m, and their height may be about 30 m. If these were caused by transverse vortices, this would suggest the depth of the flood waters when they were eroded was in the range of about 30 m. Transverse vortices would act as rollers which minimized friction at the fluid-sediment interface; they could not work very well as rollers if there were insufficient room for them in the subglacial cavity.
Erosion of lake basins was contemporaneous with streamlining
The environment of drumlin formation seems to be one in which vast areas of the continents were swept by catastrophic currents; the currents generated in those unique conditions would probably also have been capable of producing other landforms, such as erosion of lake basins and transport and deposition of sediments. Shaw and others [Shaw, Rains et al 1996] used digital terrain elevation data to extend the subglacial outburst flood hypothesis and they showed that former vast floods left their marks in the Missouri and Mississippi valleys as far away as the Gulf of Mexico. This bold thinking is commendable, and the scale of former currents which formed the drumlins should match the effects we observe. But the subglacial meltwater environment seems too limited in scope, and unsuited for explaining the excavation of rock basins of lakes such as the lower Great Lakes, and the Finger Lakes of New York, and the many other large lakes along the border of the Canadian Shield in Canada. Whereas the drumlins occur on high plateaux in western New York, and other drumlinized areas, these are incised by deep wide valleys, some of which contain lakes. Lake basins such as Lake Ontario were evidently excavated subsequent to the drumlin formation on the surrounding land since some of the drumlins along the shores were cut in half, and many others were eroded away during the excavation of the lakes. Drumlins are reported to occur beneath the waters of Lake Huron at its northern shore.
Drumlin fields are about the same age
The extent of erosion of the drumlins in existing conditions does not seem to support the notion of significant age differences between various drumlin fields. In Ireland, the drumlins resemble those of New England or Ontario; they are evidently about the same age all over the world. This supports a contemporaneous origin, rather than seperate subglacial outburst events at different times.
What about the imprint of the glacier?
The theory of meltwater outbursts forming drumlins implies the hypothetical ice sheet impressed the streamlined drift after all the floodwaters had escaped from beneath the ice. This means the great ice sheet, after the meltwater escaped, would be let down like a huge press or mould on top of the drift, that would surely have left its imprint. So the streamlined landforms, drumlins and flutings that were composed of drift, would be modified by the pressure of the remnant of the ice sheet being impressed into the sand and gravel, where there was drift at the surface. But there is little evidence of this; the streamlined surfaces are intact, and the stratified sand in many drumlins is not deformed.
C. Warren Hunt's comments on Shaw's theory focus on the mechanical plausibility of huge volumes of water accumulating beneath the hypothetical ice sheets.Shaw observed, as have others including me, that huge fields of these [drumlin] features occur. The drumlins are found side by side in fields [50 kilometers] in width and length, with [50-meter] heights and 1-to-2 [kilometer] lengths, and parallel orientation so perfect that only a massive water flow could have produced them....
An alternative to the meltwater flood hypothesisEvidence for tectonic movements, such as elevated non-horizontal abandoned shorelines, is abundant in most regions where drumlins occur. Many of these areas also feature evidence for erosion, such as lake basins. The crustal warping indicated by tilted, abandoned shorelines suggests a mechanism for the generation of the rapid currents that streamlined the land: rapid elevation of submerged areas generated the currents as the flood waters were displaced.
In the theory of drumlin formation proposed by Cox, newly deposited sediment and bedrock were streamlined by vortices in fast currents of the flood. The currents were generated by uplift, which spilled overlying flood waters to neighboring areas. The process of streamlining by longitudinal vortices, resulting in drumlins and flutings, involves both erosion and deposition; this would form rock drumlins, homogeneous in composition, formed of bedrock. In many areas tectonic adjustments were accompanied by in situ disintegration of the surface rocks, which altered streamlined surface rocks to drift sands and gravels. Penetration of the disintegration to varying depths caused drumlins of drift, some partly drift with bedrock cores, and left some drumlins unaltered as rock drumlins. All these kinds of drumlins may occur together in the same swarm, having similar morphology, and orientation, so were contemporaneous.
Where the current velocity was less, for example in basins or depressions where depth was a little greater, the longitudinal vortices which caused drumlins broke up and transverse vortices formed, that acted as rollers, causing the transverse ridges of bedrock. Examples of these occur on the Bruce Peninsula in Ontario. Disintegration of these transverse ridges would result in the drift ridges called rogen moraine. It is quite possible in this interpretation that in areas of rogen moraine, bedrock ridges having similar morphology may also be present, and examples of this may have been found in Alberta. Typically there is a transition between rogen moraine and drumlins along the flow path of the former currents.
Extreme erosion may also occur in this environment, so that areas where drumlins had formed were eroded further, and lake basins were excavated, the sediment being carried far away towards the continental shelves as the water drained off the land. This fits the evidence produced by Shaw et al from digital terrain elevation data [Shaw et al 1996].
ConclusionDifferent approaches to the question of the origin of drumlins, the flood on one hand, and the glacial meltwater flood theory on the other, have led to a convergence of ideas on the origin of drumlins and related features. This is remarkable in itself, and especially rare in the history of such a perplexed question as the origin of drumlins. In this paper several defects of the subglacial meltwater hypothesis have been identified. There are several unknowns also in the flood interpretation, such as the mechanism of the crustal uplifts which generated the currents of retreating flood waters. The questions and problems identified in this article could help to point out areas where further research will lead to a better understanding of the conditions in which drumlins were formed.
The Creation Concept | Controversy About the Glacial Theory