With regard to the glaciation of the northern part of the Great Plains, Dr. G.D. maintains that the region, as a whole, has been submerged, and that floating ice has been the main agent in its glaciation, a view which is not in accord with that of the majority of American geologists.Dawson’s conclusion was likely based on the distribution of silt sediments and elevated shorelines, later interpreted as beaches of ice dammed lakes, especially Glacial Lake Agassiz, by W. Upham.
Some of the most impressive evidence for large scale catastrophic flooding is the streamlined drumlin field located in the Livingstone Lake area of northwestern Saskatchewan, that was described by Shaw and Kvill . Shaw and Kvill proposed a mechanism for flooding involving subglacial melt water that accumulated beneath a hypothetical ice sheet.
According to their theory, the pressurized subglacial water suddenly burst out, releasing a powerful flood, driven by the pressure of the overlying ice sheet. The currents eroded and streamlined the rocks and drift beneath the ice, forming drumlins and other features over wide areas. Their proposed mechanism has since been applied in several other areas, including the Avalon Peninsula of Newfoundland; the northeastern shore of Georgian Bay, Ontario; the basins of Lake Ontario and Lake Erie; the drumlin fields of northwestern New York; Lake Michigan, and the interior of British Columbia where subglacial meltwater is believed to have contributed to the great Lake Missoula flood in Eastern Washington, first described by J.H. Bretz.
An objection sometimes raised against the glacier meltwater flood theory is that the volume of water that could accumulate beneath the hypothetical ice sheet would be inadequate for floods with the immense volumes estimated by Shaw and his coworkers.
The alternative to the subglacial meltwater flood theory, that is supported by the writer, does not have this limitation, and does not require an ice sheet "lid". Instead, currents were initiated by tectonic uplift of submerged lands. Streamlined landforms indicate the course of the currents generated by the uplifts, so we can infer the general source areas. Crustal uplifts in the northern part of the Canadian Shield probably generated the currents that formed the Livingstone Lake area drumlins.
Where differential uplifts involved very large areas, the volume of the floods that flowed over areas peripheral to the uplifted areas could be very large, because current velocity over any part of the perimeter would depend upon the volume (square of the radius times depth), while the length of the perimeter depends on the radius. If currents had been restricted due to land rising above the surface, the current velocity in those confined areas would be increased.
Currents flowed from the Shield towards the southwest over the Livingstone Lake area, and were diverted towards the north and south at the foothills of the Rocky Mountains. Evidence of these catastrophic current flows is found in various parts of the province of Alberta. For example, Munro and Shaw proposed that subglacial meltwaters caused a flood that formed streamlined hills and transverse ridges (rogen moraine), hummocky terrain, etc., in south-central Alberta. [Munro and Shaw 1997]
|Map showing location of the giant current ripples on the Milk River Ridge in southeastern Alberta. The canyon of the Milk River transects the pattern of giant current ripples.|
The ridges are up to 25 m high, with relief decreasing to 1-2 m in the northwest. The crest to crest wavelength was 405 m in the northwest, increasing to 1,300 m in the southeast. Flutings oriented northwest to southeast were superimposed on the ridges. Troughs between the ridges contain wetlands and small elongated lakes.
Estimated flow rates during the formation of these features were 106 to107 m3/sec. Water depths of from 10 and 60 m were invoked. The mechanism for the formation of these features proposed by the writer is similar to that described by Beaney and Shaw but without the ice sheet "lid." Again, in my theory, these features are consistent with a flood event generated by tectonic uplift of submerged regions, either in the area of Hudson Bay or now exposed in the Canadian Shield.
References:Beaney, C.L. and J. Shaw, 2000. The subglacial geomorphology of southeast Alberta: evidence for subglacial meltwater erosion. Canadian Journal of Earth Sciences, 37, 51-61.
Coleman, A.P. 1941. The Last Million Years, U. of Toronto Press.
Cox, D.E. For a critique of the subglacial flood mechanism, see: Drumlins and subglacial meltwater floods. Additional information can be found in: The Glacial Paradox. (PDF file)
Munro, M. & Shaw, J. 1997. Erosional origin of hummocky terrain in south-central Alberta, Canada. Geology, 25(11) p. 10277-1030.
Shaw, J. & D. Kvill 1984. A glaciofluvial origin for drumlins of the Livingstone Lake area, Saskatchewan. Canadian Journal of Earth Sciences 21, 1442-1459.
Winslow-Spragge, L. 1962. Life and letters of George Mercer Dawson 1849-1901
© 1999, 2001 by Douglas E. Cox
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