Bedrock Rafts and Megablocks in the Drift

The occurrence of these large bedrock masses amid the Pleistocene drift is disconcerting.
- A. MacS Stalker

Introduction
Examples of bedrock "rafts" or megablocks
Interpretation as effects of in situ disintegration
References
Links

Introduction

Bedrock rafts in the drift, or megablocks, (a.k.a. glaciotectonic features) are slabs of bedrock resting on layers of drift, often buried by other drift. Some are quite extensive, covering areas of up to several hundreds of km². They may be deformed, or associated with deformation of the surrounding drift. As noted by Stalker in the quotation above, and in his discussion included in the following section, they are disconcerting or enigmatic in therms of a glacial interpretation, because extensive, thin rock slabs would readily break up if they were subjected to the forces required for transportation. A much simpler explanation is available and is proposed below.

This next section of this article lists some links to Web sites and published articles where bedrock rafts and megablocks are described and illustrated, and provides some brief quotations from some of the sources. There are also direct links to photos illustrating the bedrock rafts at some of these sites. The images will open in a new window. Use the browser's "Back" arrow to return to this page.

Examples of bedrock "rafts" or megablocks

A "raft" of bedrock consisting of chalk, in boulder clay at Holderness, Yorkshire, UK is illustrated here, from Friends of the Speeton Clay. Geologists claim that a former ice sheet extracted a thin, fragile slab of chalk from the bedrock of the North Sea, and deposited it within the boulder clay! A second photo is available here.

Rafts or "Schollen" consisting of chalk occur at Kvarnby, in southern Sweden, over about 4.5 km in a zone 700-800 m wide. The dimensions of the chalk rafts were reported to be up to 700 - 800 m long and 300 m wide, with a maximum thickness of 30 m. Some were apparently intersected by layers of till. The total thickness of the drift in the area is between 20 and 60 m. They are covered and are underlain by clay "till", and some of them are the sites of quarries. They were described by Bertil Ringberg in Glacial Deposits of North-West Europe, and he suggested they were transported in the base of an ice sheet about 25 km northward, from their original location at the bottom of the Baltic. Ringberg wrote:

The large chalk rafts are presumed to have been transported frozen to the basal ice from the sea floor of the Baltic, at least 25 km northwards to Kvarnby. Larger rafts were probably divided into smaller ones during the deposition. This would imply that the ground was frozen to a depth of at least 30 m before the rafts were torn away from the bedrock in the Baltic.

Stacked bedrock rafts in the Sperrin Mountains of Ireland were mentioned in a 2001 AGU meeting report. Author J. Knight stated:

Stick-Slip Mechanism of Basal Ice
In the southern Sperrin Mountains subglacial sediment sequences comprise tabular bedrock rafts which are interbedded with diamicton (till) and brecciated bedrock and separated by glaciotectonic shears. Bedrock rafts and diamicton beds alternate laterally and vertically in the sediment profile, suggesting that the ice-bed interface was chequered spatially with both high-strength (rock rafts) and low-strength (diamicton) patches during accumulation of the sediment pile.

A bedrock raft of sandstone resting on drift that was exposed in quarry near Freienwalde, northeastern Germany, is illustrated here. The caption says: "The light-colored mass at top is a bedrock raft (megablock) of Oligocene glimmersand (micaeous sandstone) that was thrust over the brown-colored glacial sediment underneath." [The photo is from a website of J.S. Aber, at: http://academic.emporia.edu/aberjame/ice/lec08/lec8.htm.]

Another site by J.S. Aber which includes numerous photos of bedrock rafts and megablocks is: http://academic.emporia.edu/aberjame/ice/lec16/lec16.htm. Aber wrote:

The northern Great Plains is one of the world's premier regions for glaciotectonic structures and landforms. All manner of composite ridges, hill-hole pairs, and megablocks are found from North Dakota to central Alberta. Several factors contributed to widespread glaciotectonism: ice advance upslope against topographic barriers, poorly consolidated bedrock containing confined aquifers, proglacial lakes, and surging activity of ice lobes (Aber et al. 1995).

Exceptionally large ice-pushed ridges are developed along the Missouri Coteau of southern Saskatchewan and eastern Alberta--see Fig. 16-5. The Dirt Hills and Cactus Hills are among the best developed ice-pushed ridges in the world (Aber 1993a). They are located on the Missouri Coteau upland southwest of Regina in southern Saskatchewan. The highest elevations of the Dirt Hills exceed 2880 feet (880 m), more than 1000 feet (300 m) above the Regina Lake Plain immediately to the north, and 400 feet (120 m) above the Missouri Coteau upland to the south.

Aber notes that "ridges are composed primarily of till that contains deformed floes of lacustrine sediment." He wrote: "Uplifted sandstone bedrock of the Belly River Beds forms the cores of some ridges."

Many examples of large scale megablocks or bedrock rafts are mapped in the Quaternary Geologic Map of the Winnipeg 4° x 6° Quadrangle. A large concealed bedrock raft in Saskatchewan, with dimensions about 30 km x 38 km, located at 102°W, 50°50'N, is indicated on the western edge of the map. Aber (1989) estimated its volume to be about 60 cubic km, and its area 1000 square km. He referred to it as the Esterhazy megablock. It lies between the towns of Esterhazy and Rocanville, Saskatchewan. The megablock is divided by the Qu'Apelle valley. It consists of claystone from the Cretaceous Riding Mountain Formation and overlies the same formation, apparently with a layer of drift underneath, separating the megablock from the lower rock formation from which it was derived. A test well drilled near the western end of the megablock encountered 2 m of drift after penetrating 80 m of brecciated and mylonitic bedrock. The maximum thickness of the megablock is 100 m and it is about 380 times longer and 300 times wider than its thickness. Aber wrote:

The only conceivable means of displacing a megablock of such size was by freezing onto the bottom of an overriding ice sheet, in which case the megablock became the basal layer of the ice sheet. It is highly improbable, given its dimensions, that this megablock could have been pushed in front of an advancing glacier, whether it was permafrozen or not. Subglacial sliding of a permafrozen slab over a thawed substratum seems to be the most likely explanation for displacement of the Esterhazy megablock.

The subsurface cross section and bedrock contour map provided by Aber both show that the megablock forms a cap over a raised area of bedrock of the Riding Mountain Formation. The interpretation of the structure as a megablock appears to be based studies of groundwater resources in the area.

This map shows the Esterhazy megablock, composed of the Odanah Member of the Pierre Shale, in gray. The Odanah Member is a hard, siliceous, light-gray shale. A layer of Pierre Shale overlies the Odanah Member in the western part of the southern section of the megablock. The Pierre Shale (formerly known as the Riding Mountain Formation) in the area is a thick, soft, gray noncalcareous silt and clay, that has not been consolidated into solid rock, with the exception of the Odanah Member. The map was prepared by the writer from the Geological Atlas of Saskatchewan. The towns of Esterhazy and Rocanville are shown, along with roads and rivers. The border of Saskatchewan and Manitoba is the edge of the green on the right side of the map. The other lines represent drumlins or flutings on the drift surface.

Section showing the structure of the Esterhazy megablock

The above section shows the structure of a portion of the Esterhazy megablock based on well log data. The megablock consists of the Odanah Member of the Pierre Shale, shown above the dark horizontal line, which was added by the writer to show the approximate location of a layer of drift and disturbed shale beneath the Odanah Member, adapted from a suburface cross section (Figure 8, p. 291) in J.S. Aber (1989). The section by the Saskatchewan Soil Survey, 1987 omits this information. The vertical exaggeration is 20x.

Glacio-tectonics is an MS PowerPoint presentation by D. Sully, N. Searle, S. Smith and T. Saunders about bedrock rafts or megablocks in the drift. The URL is: http://www.ex.ac.uk/geography/modules/GEO2209/Glacio-tectonics2.ppt. The authors attribute megablocks in the Dirt Hills and Cactus Hills of southern Saskatchewan to glacial thrusting, a process which they believe resembles "continental collision and mountain building at plate boundaries." They wrote:

Composite Ridges and Thrust Block Moraines
· The Dirt Hills and Cactus Hills, Missouri Coteau uplands, southern Saskatchewan, Canada.
· Dirt Hills and Cactus Hills are large composite ridges formed by the advancing of ice lobes from the Laurentide ice sheet which folded and thrusted the bedrock.
· The hills rise up to 150 metres above the Missouri Coteau and cover and area of approximately 1000 km².
· The ridges are arcuate in shape and are parallel to each other with narrow valleys in between.
· The ridges are made of folded Upper Cretaceous bedrock. A layer of glacial sediment covers most of the ridges, except in the southern Dirt Hills.

Remarkable bedrock masses in the drift in the area of the Oldman River, near Lethbridge, Alberta. An example from the area is shown in this photo from Land Forms Along The Oldman River by University of Lethbridge geography students, who write:

The Oldman Valley, at Lethbridge, contains a number of quite visible glacial landforms. None, are perhaps more visible then those of mega blocks. Look (click) on the picture above, can you see the light grey slab of sandstone bedrock, sandwiched between the tills? This particular landform is called the Laundry Hill mega block. Mega blocks, are an unusual feature. The Laundry Hill mega block, was sandstone, from the exposed Oldman Formation northwest of the city and transported south to this location by glaciers. It was then dislodged as the glacier approached the valley. Geomorpholigists, such as Renee Barendregt, suggest that the mega block on Laundry Hill, may have been 1 square kilometer in size. Mega blocks are essentially formed when glaciers are able to dig large slabs of bedrock, and transport them to new locations.

The megablocks in the area near Lethbridge were described in a 1962 Geologic Survey of Canada paper by A. MacS. Stalker, the geologist quoted at the top of this page. Stalker wrote [p. 5-6]:

These deposits are remarkable for the substantial quantity of Cretaceous bedrock contained as immense blocks in them, and for the presence of other sediments containing small amounts of organic matter. The large masses of Cretaceous bedrock are a puzzling feature of these intertill deposits, and their presence is as yet completely unexplained. The mention of these bedrock blocks solely in the descriptions of Sections 1 and 3 does not do justice to their widespread occurrence. They are present also in exposures near Section 2, and in practically every exposure along Oldman River between Section 3 and the junction of the Bow River, a distance of about 100 miles (about 60 miles in a straight line). In some places a continuous block of this intertill bedrock is more than a mile ling, and in some instances may possibly extend continuously for several miles. In addition there is indication from drill-hole logs that one of these large masses 8 to 10 miles north-northeast of Taber may be spread over several square miles and be up to 90 feet thick, though generally much thinner. Commonly, and perhaps in most exposures, the bedding in these masses is approximately horizontal or only slightly deformed, but elsewhere there is much contortion. The large bedrock masses are thought to be in equivalent stratigraphic positions in all exposures, and thus they form an extremely valuable and easily recognized marker horizon.

The occurrence of these large bedrock masses amid the Pleistocene drift is disconcerting. Slumping takes place with great ease in the soft, easily lubricated bedrock of the area,, but this is not thought to be the cause of their displacement. Slumping should cause more contortion and deformation of the beds than is found, and such extensive but relatively thin blocks would hardly be expected to hold together under such a method of transportation. The chief argument against slumping as then agent of transportation is the apparent lack of proper conditions of gradient necessary to cause such slumping. Shove or pushing by an ice-sheet also seemingly would not allow such widespread thin blocks to retain their shape or even hold together during movement. The most attractive hypothesis, even though not satisfactory, is that these masses were dragged beneath an actively moving glacier into their present positions. The top of the bedrock may have become attached to the base of the ice-sheet by freezing, pulled away from the underlying bedrock, and then slid over the underlying slippery shales and tills to be deposited when the glacier could drag it no farther. A high water-table, such as would likely be present under glacial conditions, could have aided the process by increasing the lubrication of the underlying deposits. There remains the fact, however, that in many places are overlain by alluvium (e.g., Section 3), whereas if they had been dragged into place beneath the ice, they would presumably be overlain by till. The question as to their means of transportation cannot yet be answered.

A large erratic composed of quartzite in Southern Alberta, near the town of Okotocks, known as the Okotoks Erratic, or "Big Rock," is part of the Foothills Erratics Train, a line of quartzite boulders thought to have been carried from the mountains in Jasper National Park by former glaciers. How could former glaciers or ice sheets have transported such an enormous mass, and deposit it in its present location?

Glaciotectonics or Glacial tectonics, at http://pbisotopes.ess.sunysb.edu/reports/dem_2/glaciotectonics.htm discusses bedrock rafts and related features in the drift of New York, Connecticut, Massachusetts, and Rhode Island.
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Interpretation as effects of in situ disintegration

The largest megablock in the Winnipeg 4° x 6° Quadrangle area was about 30 km x 38 km and probably covered an area of about 1,000 km². The map indicates that thickness of the megablocks ranges from 10 to 100 m. Very extensive, thin slabs of bedrock overlying drift seem to require that the drift below them must have formed by an in situ disintegration mechanism.

The presence of huge slabs of intact bedrock in the drift seems enigmatic and strange in the glacial interpretation. It has been a source of embarrassment to glacialists such as Stalker, who called their presence disconcerting. (See quote at the top of the page.) The glacial theory does not easily accommodate or explain the emplacement of large slabs of rock over drift supposed to have been deposited either directly from ice, or when the proposed ice-sheet finally melted. Where the width to thickness ratio for the bedrock rafts is very large, the idea of glacial transport and deposition is rendered highly unlikely and impractical.

A mechanism for forming similar structures does not seem to exist in existing ice sheets. The presence of bedrock slabs, hundreds of square km in area, being transported in the base of ice sheets in Greenland and Antarctica would show up in seismic profiles, and should be rather easy to detect.

The idea of in situ disintegration involves an alteration of sediments, resulting in formation of drift in place, whether it occurs as a mantle over bedrock, or sandwiched between two layers of bedrock, as might occur where an upper bedrock layer was left partly intact as disintegration occurred below. Formation of layers of drift below a layer of bedrock, by the in situ disintegration process, could result in bedrock rafts or megablocks of almost any extent, since in this explanation there is no need for the huge slabs of bedrock to have been torn away from the underlying bedrock and transported to their present locations.

The lateral dimensions of bedrock rafts would probably be limited by the erosion of valleys that cut through them, in the environment of catastrophic erosion due to the currents of retreating flood waters. Also, expansion effects that accompanied the formation of the drift underlying megablocks may have caused lateral movements which could possibly cause extensive rafts to break up into smaller ones.

The sand, clay and gravel of the drift is explained by a disintegration of the sedimentary rocks, during former catastrophic conditions, that accompanied pressure release as overburden was eroded by currents, exposing fresh rock surfaces to lower pressure, and a stress gradient, in which the components of the rocks responded to the altered conditions. The process formed layers as it penetrated from the low pressure surface to the higher pressure.

Chemical changes driven by a high stress gradient in the vicinity of the low pressure surface resulted in disintegration of the rock, typically forming layers of sand, gravel and clay. The stress gradient would drive fluids from the rock and promote crystallization and dehydration processes. The disintegration continually exposed new surfaces, where the same process continued, resulting in the formation of layers of sand and gravel.

Several reports indicate megablocks consist of sandstone, which apparently remained intact while disintegration occurred below, perhaps because of high porosity, or because of the presence of extensive joints which allowed fluids to escape to the surface from lower levels. These megablocks vary in thickness, and generally remain horizontal or slightly tilted, but could be deformed due to the contortion of surrounding drift, caused by expansion effects.

In limestone areas where limited subsurface disintegration occurred, typically along joints, cave systems could be initiated by a similar process, and develop as the drift and clay fill was eroded by groundwater streams.

Formation of the Esterhazy megablock

While the sediments in the vicinity of the Esterhazy megablock were under hydrostatic stress, an uplift event generated currents which eroded the sediments in the area. Vertical stress was removed by the rapid erosion of overburden. This put the megablock and the surrounding rock under net horizontal compressive stress. Disintegration converted rock surrounding the megablock to drift. The disintegration penetrated deeper on all sides of the megablock, which forms a cap over a raised bedrock platform.

Horizontal compression in the surrounding drift was maintained, because the disintegration was accompanied by expansion. Currents of the retreating flood waters above eroded a valley in the drift, right through the middle of the megablock. This became the Qu'Appelle valley. Erosion and removal of drift in the valley exposed underlying rock to lower pressure so there was further disintegration. The valley was eroded deeper. The erosion removed much of the drift that filled the valley. Meanwhile disintegration also occurred in the buried Rocanville valley south of the megablock, which is roughly parallel to the Qu'Appelle valley, but it was not eroded. The disintegration penetrated to an even greater depth in this buried valley, which was much wider than the Qu'Appelle valley. The depth of the drift here is about 250 m (Figure 8 in Aber, 1989). When the disintegration was penetrating downwards in the buried Rocanville Valley, conditions in the adjacent rock at a particular level were apparently suitable for the disintegration to occur, and so a thin layer of drift was formed at that level, that separated the Odanah Member of the Pierre Shale from its base, forming what is called a megablock.

At the base of the drift overlying the Odanah Member in the section illustrated above a layer of disturbed shale is indicated. This disturbance may have resulted from expansion effects in the overlying drift, that spread outward from thicker regions towards areas where the drift layer was thin. A similar expansion effect may have caused disturbance in the shale underlying the subsurface drift layer beneath the Odanah Member. The drift itself may have moved, due to expansion effects, rather than the megablock! If there was any movement of the megablock, it must have been very limited. As the Qu'Appelle valley was excavated by currents, some of the drift below the Odanah Member could have moved towards the valley, where horizontal stress was relieved. The weight of the Odanah Member and the drift above it would tend to sqeeze the buried drift layer out towards the valley. This could possibly cause deformation effects in the underlying shale such as mylonite, faulting, folding, breccia, slickensides etc, that were reported by Aber in the vicinity of the Qu'Appelle valley and in drill holes.

The in situ explantion of the Esterhazy megablock above is much simpler than the glacial explanation. The glacial interpretation is vague in comparison, and seems unnatural and strange. Transport of such a large slab of rock tens or hundreds of km as proposed in the glacial theory would have left an enormous groove in the bedrock, but no such groove exists. The idea of glacial transport of such a large slab of rock seems like a tall tale.

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References

Aber, J.S. 1989. Spectrum of constructional glaciotectonic landforms. In: Goldthwait, R.P. and Matsch, C.L., Eds. Genetic Classification of Glacigenic Deposits: Final Report of the Commission on Genesis and Lithology of Glacial Quaternary Deposits of the International Union for Quaternary Research (INQUA). A.A.Balkema, Rotterdam. pp. 281-292.
Aber, James S., Croot, David S., and Fenton, Mark M., 1989. Glaciotectonic Landforms and Structures. Kluwer Academic Publishers, Dordrecht.
Aber, J.S., Bluemle, J.P., Brigham-Grette, J., Dredge, L.A., Sauchyn, D.J. and Ackerman, D.L. 1995. Glaciotectonic map of North America, 1:6,500,000. Geological Society of America, Maps and Charts Series, MCH079.
Benn, D.I. and Evans, D.J.A. 1998. Glaciers and Glaciation. Arnold.
Boulton, G.S., 1986. Push-moraines and glacier-contact fans in marine and terrestrial environments, Sedimentology, p677-698.
Clayton, L., Teller, J.T. and Attig, J.W. 1985. Surging of the southwestern part of the Laurentide Ice Sheet. Boreas 14: 235-241.
Ehlers, J, ed. 1983. Glacial Deposits of North-West Europe. A.A. Balkema. Rotterdam.
Knight, J., 2001. Field Geological Evidence Supporting a Stick-Slip Mechanism of Basal ice Flow From a Late Pleistocene Ice Sheet. AGU 2001 Fall Meeting.
http://www.agu.org/meetings/fm01/fm01-pdf
Kupsch, W.O. 1962. Ice-thrust ridges in western Canada. Journal of Geology 70:582-594.
Ringberg, B. 1983. Till stratigraphy and glacial rafts of chalk at Kvarnby, southern Sweden. In: Ehlers, J. Op.Cit. p. 151-154.
Saskatchewan Soil Survey, 1987. The soils of the Moosomin, Martin, Rocanville and Spy Hill Rural Municipalities Nos. 121,122,151,152
Canada National Land and Water Information Service, SKS207.
http://sis.agr.gc.ca/cansis/publications/sk/sks207/intro.html
Stalker, A. MacS. 1963. Quaternary stratigraphy in Southern Alberta. Geological Survey of Canada, Paper 62-34.
USGS, 2000. Quaternary Geologic Atlas of the United States, Winnipeg 4° x 6° Quadrangle, U.S. and Canada, Miscellaneous Investigations Series, Map I-1420 (NM-14).
http://pubs.usgs.gov/imap/i-1420/nm-14/I-1420_nm-14_screen.pdf

Links

Streamlined Rocks
Pothole at George Lake
Potholes in the Susquehanna River
Controversy about the Glacial Theory