Great Lakes Origin by Diastrophic Processes

Contents

Formation of the Drift
Drift Composition
Erosion Aided by Disintegration
Interglacial Sediments and the Disintegration Theory
References

Formation of the Drift

Photo showing configuration of pebbles in drift.In the disintegration interpretation, the formation of the drift was caused by rapid erosion of overburden. As fresh sediment or rock surfaces were exposed, chemical adjustments such as precipitation of new minerals, replacement, lithification and crystallization occurred in the sediments, as the mix of minerals in the sediment adjusted to the new conditions. In some areas, because of these processes, rapid disintegration occurred. Sand and gravel formed, as shown in the photo. Rounded surfaces of pebbles minimized surface area. The components of the sediment became organized into concretions which turned into  pebbles as the matrix around them was disintegrated. The disintegration was an in situ process of alteration of bedrock that penetrated down to various depths. The process formed layers, with characteristic patterns of cross stratification. In this non-sedimentary process, embedded pebbles in the cross strata would not exhibit stoss and lee effects, or "current shadows" in the form of long ridges on the lee side of pebbles, or horseshoe-shaped troughs on the stoss side where the current was deflected by obstacles. On the other hand, turbulence effects such as this would be expected to occur around the embedded pebbles if these layers were formed in an environment of deposition from fast currents.

The photo shows a typical exposure of stratified drift gravel. No evidence of directional "current shadows" in the vicinity of pebbles is seen. Although pebbles occur in the layers, there is no indication of turbulence, and the strata is not "bumpy" due to the presence of any elongated lee-side ridges of other pebbles, but is smooth and distinct. This configuration, with smoothly curved, distinct laminations of patterns of cross strata, is typically found  in the drift of kames and eskers, and may occur within drumlins, and is probably also found in the drift of rogen ridges.

Flume experiments and observations of sedimentary processes indicate that as sediments are transported in an environment of fast currents, local “particle clusters” occur. These are groups of particles that have accumulated in stable sites for deposition. The pebbles and sand grains find stable pockets among their neighbors, behind an obstacle or a cluster of other pebbles.  Whiting wrote about this phenomenon in real sediments as follows [Whiting, 1996, p. 212]:

As the “birds-of-a-feather” expression suggests, clusters form because the constituent grains find very stable pockets among their neighbors. In some cases, the grains can remain on the bed only because these coarse grains sit adjacent to one another. Clusters in turn can act to modify the flow field and themselves lead to disentrainment of other size fractions. Current shadows of finer sediment form in the lee of these obstacles. Fine grains accumulate in the complex wake until the bed in the lee of the clast has been so smoothed that additional grains are either unable to find a stable pocket, or flow is altered by this shadow and finer grains are moved past the obstacle. In other cases, sediment may accrete in the front of the clusters. These static accumulations are possibly seeds for the development of other sorted features such as migrating bedload sheets.
The formation of "current shadows," clusters of pebbles in streamlined accumulations, and their removal from areas between clusters should lead to a distribution of pebbles in the gravels in clusters having linear orientation. The pebbles would tend to be aligned behind the larger ones. One would suppose that this alignment of pebbles in clusters would provide a reliable directional indicator in cross stratified gravels and in sandstones, for example. But to my knowledge, techniques based on this phenomenon are not used by geologists in their studies of the drift, or cross stratified drift sands, or gravels and conglomerates. Instead, the orientation  of the long axes of pebbles has been considered diagnostic of current flow direction in "till fabric" studies. But these studies often yield misleading results.

Whiting also describes observed periodic accumulations of coarser grains in thin bands transverse to the current direction, which he calls “bedload sheets” [Whiting, 1996, p. 217]. These form due to sorting effects of the current, where the grains and pebbles are of different sizes. Wavelengths of from 0.5 to 2 meters occur in gravel. To my knowledge, structures in the drift corresponding to these have not been reported.

The lack of directional indictors near pebbles of the drift, and the absence of bumps in the laminations where pebbles were present, is a crucial observation on which the disintegration theory rests, as it is regarded as diagnostic of a non-sedimentary origin. Layers of sand could have formed by successive surfaces of disintegration penetrating downwards through a lithified sediment, forming distinctive patterns.

Expansion of the disintegration product resulted in movements of the drift over its subsurface causing patterns of striations on bedrock below the drift, or over other drift layers. Movements due to expansion can explain scratched boulders, distorted layers, and faults in the drift.

In the nineteenth century Raphael Pumpelly found evidence for a mechanism of in situ disintegration of rock, to considerable depths, and he proposed the subsequent erosion of unconsolidated material formed rock basins such as the Great Lakes. Like J. W. Dawson, Pumpelly understood the limited capacity of former ice sheets to break up the rocks beneath them. Pumpelly thought of the disintegration mechanism acted over long time spans.

Where the disintegration occurred in areas previously streamlined by the currents, it modified the composition of drumlins and flutings developed in unonsolidated sediment that had become lithified. Since the depth of alteration varied, some drumlins were completely disintegrated, some only partially, and some remained  unafffected by disintegration. This resulted in the drumlins  having variable composition, although others in  the vicinity may have similar size, and shape, and orientation.

Where the disintegration penetrated to considerable depths, and where the lateral expansion was restricted, the expansion effects thrust up pressure ridges. This explains the formation of kames and eskers in the disintegration theory. The so-called "moraines" that have been identified in the Great Lakes region by glacialists are not true glacial moraines at all; they are mounds and ridges of drift that was displaced laterally in some places because of expansion effects as the drift was formed. In this interpretation the orientation of striations on bedrock caused by drift movements due to expansion would not need to correspond to directional features caused by currents. Crossing patterns of striations could form because of different centers of expansion. Some patterns of striations may have been due to the currents, and these would tend to parallel to the streamlined landforms and flutings in bedrock.

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Drift Composition

The drift contains pebbles and boulders of many varieties. In the glacial theory, the rocks that are different from local bedrock (called  erratics) are thought to have been transported from distant regions by ice. In the disintegration interpretation many of these “foreign” pebbles and boulders must have formed by metamorphic processes in place during disintegration. However, some of the drift pebbles and  boulders may have been transported by powerful currents and redeposited.

Most of the drift resembles local bedrock; a detailed study of drift composition across the boundary of the Shield and Paleozoic sedimentary rocks in central Ontario (Cogley et al, 1997) found seven eighths of the pebble fraction was local, from within 2-5 km.

Generally far too many varieties of pebbles are present in the drift, for a glacial interpretation to work, as the possible source rocks inmany regions are few compared to the variety of pebbles. The variability in both the composition and texture of pebbles in the drift is explained in the disintegration theory by the effects of chemical processes that accompanied disintegration.

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Erosion Aided by Disintegration

Disintegration penetrated vertically, forming vertical cliffs of escarpments. Some escarpments remain hidden by the drift cover, for example portions of the Onandaga Escarpment, parallel to the Niagara Escarpment, which extends from Fort Erie to west of Brantford, Ontario, are completely buried by drift.

Buried valleys and canyons in the bedrock are common. One of the deepest of these is the buried Dundas Valley, which forms a re-entrant valley in the Niagara Escarpment at the western end  of Lake Ontario. It is filled with drift to a depth of 600 feet near Dundas, and its overall depth is 1,000 feet. (Karrow, 1973.) The excavation of a buried gorge formed the gorge of the Niagara River. An unexcavated section in the vicinity is known as the buried St David Gorge. Other similar examples occur at Elora Gorge, Rockwood, and the Nassagaweya Canyon near Milton. Some of the buried valleys intersect, and the patterns may be due to joints in the former bedrock.

Disintegration of bedrock occurred during the erosion of lake basins. Currents swept away the drift as it formed, exposing fresh bedrock, which enhanced the process of disintegration and the excavation of the lake basins.  This may have been an especially important factor in regard to the excavation of the rock basin of Lake Superior,  which is formed in granite.

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Interglacial Sediments and the Disintegration Theory

"Interglacials" are invoked by the glacialists for explaining away evidence contrary to the climatic conditions necessary for glaciers to exist. The evidence for interglacials is usually the presence of organic rich sediments or buried soils between layers of gravel and sand. The fossils consist of wood, leaves, bones of mammals, insects, and shells. Many of the animals represented are now extinct. The fossils often indicate much warmer climates existed, incompatible with glacial conditions, for example animals such as the ground sloth, camel, saber-toothed tiger, and giant beaver, in North America. Warmer "interglacial" periods are proposed, as many as needed, when the hypothetical ice sheets presumably entirely melted away, and later returned.

There is little evidence for crustal "rebound" associated with melting away of these hypothetical former ice sheets. Was the crust of the earth depressed hundreds of meters each time the ice sheet returned? What about evidence for systems of spillways as meltwater from previous ice sheets flowed towards the ocean?

How are the so-called interglacial sediments explained in the disintegration theory? There was considerable erosion of the drift by powerful currents in some areas. Deposition of layers of drift eroded from elsewhere upon a drift mantle might explain interglacial sediments. The upper drift layers could have been transported by currents and deposited in valleys or depressions, or in deltas formed above previously formed drift. Also, disintegration might have sometimes occurred beneath unconsolidated sediments containing various kinds of fossils.

In the disintegration theory, animals and plants represented in the interglacial sediments could have been transported by currents and so might include animals from various climate zones.

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References

Cogley, J.G., M. Aikman, and D.J.A. Stokes, 1997. Allochthonous sediment in till near a lithological boundary in central Ontario. Geographie Physique et Quaternaire v. 51 (1)

Karrow, P.F. Bedrock topography in Southwestern Ontario: a progress report. Geol. Soc. Canada, Proc., v. 25, 1973. 67-76.

Whiting, P.J. 1996.  Sediment Sorting over Bed Topography. pp.  203-228 in: Advances in Fluvial Dynamics and Stratigraphy, P.A. Carling and M.R. Dawson, eds. John Wiley & Sons, New York.


© 1999 by Douglas E. Cox
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