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The Peterborough drumlin field is located in Southern Ontario, on the northern shores of Lake Ontario. It encompasses an area of 5000km2, and contains over 1000 individual drumlins. The formation of this field occurred during the late Wisconsin Glaciation, during the readvance of the Laurentide Ice sheet approximately 13,000 years before present (Boyce & Eyles, 1991).

The following case study of the Peterborough Drumlin field is based on a study conducted by Joseph I. Boyce in 1990. Boyce (1990) studied the exposure of the Peterborough drumlin field identifying zonation of distinct drumlin types in this region. Thirty-five drumlin exposures were examined, of which twenty were logged in detail. Boyce (1990) considered grain size, degree of internal organization, the presence of sedimentary structures as well as bedding contacts in his investigation.

Figure 1 - Google Maps, 2008 - A terrain map Displaying the Peterborough Drumlin field. (Accessed Nov 17, 2008). As seen, drumlins extend

LOCATION

The Peterborough drumlin field is located north of Lake Ontario, covering an area of 5000km2. It covers regions of the Lowlands between the Niagara Escarpment and the Algonquin uplands, encompassing a belt of southwestern trending glacial terrain spanning 140km east of Lake Simcoe. The belt has an extent of 20-60km north-south, which is bounded by two moraine ridges which lie west-east - the Drummer in the North, and the Oak Ridge Moraine in the South. The Peterborough drumlin field is thought to have been formed approximately 21-13,000 years before present during the Wisconsin Glaciation (Boyce 1990).

DISTRIBUTION

The distribution of the drumlins in the Peterborough field indicates the presence of two active ice lobes in Ontario during the Wisconsin Ice Age. West of Port Hope, drumlins show a northwestern trend, measuring at an orientation of approximately 310-330 degrees;. This records a westward thrusting and radial spreading of the Ontario lobe landward out of basin (Putnman & Chapman, 1943). East of Port Hope, an abrupt shift to the south easterly direction at 190-200 degrees is noticed. Here, alignment of drumlins is consistent with those north of the Oak Ridge Moraine in the Peterborough drumlin field (Boyce, 1990). This records movement of the Simcoe lobe to the southeast, implying that the eastern and western halves of the Simcoe lobe acted independent of each other during the late Wisconsin glaciations (Boyce, 1990).

The extremely northern edges of the Peterborough drumlin field is marked by drumlins with predominantly bedrock cores, however, these drumlins are few in numbers. The northern third of this field is made up of till cored drumlins that transition southwards to stratified sediment cored drumlins. South of the transition zone, are located drumlins composed of horizontally bedded sands and gravels with minor silt and clay beds (Boyce, 1990).

Figure 2 - Showing the distribution, and orientation/reorientation of the Peterborough drumlin field, seen in the southwest (Boyce and Eyles, 1991).

MORPHOLOGY

In the northern reaches of the field drift thickness is less than 10m. Spindle shaped drumlins, composed entirely of diamict are dominant. Length-width ratios in excess of 6:1, with bilateral symmetry are most common. Drumlin diamict cores vary between 5-10m thickness and thin rapidly down the drumlin flanks (Boyce, 1990).

With drift thickness increasing in drumlins found further southwards, less streamlined ovoid drumlin forms accompany, with a diamict mantle imposed over the stratified sediment. Down the flanks of the drumlins, the capping till layer thickness generally increases. Between the drumlins and the swales, thick till, at greater than 20m can be found. Drumlins in this region are generally closely spaced together, have low amplitudes and gently sloped profiles. South of the Oak Ridge Moraine, drumlins have axial ratios that are less than 3:1 and are thus described as streamlined hills or drumlinoids.

COMPOSITION

As noted by Gravenor (1957), 3 types of drumlins can be identified in the Peterborough Field.

1. Drumlins with a rock core and till cap of variable thickness,
2. Drumlins composed entirely of till and
3. Drumlins with a core of stratified glacial fluvial/glaciolacustrine sediments with a till cap.

Drumlin sediments can also be divided into three main types based on the texture and degree of sorting (Boyce, 1990):

• Diamict Facies: This type of sediment is typically matrix supported, composed of unconsolidated subangular to rounded clasts which range from fine gravel to boulders. Basal-Diamict contacts are mostly erosive and disconformable, exhibiting levels of incorporated subjacent material. Diamicts can be further divided based on internal organization into massive and stratified diamicts (Boyce, 1990).
  • Massive diamict are texturally homogenous and show little evidence of internal stratification and grading, occurring as planar beds, with uniform thickness. They are the most important type of diamict in both till and stratified cored drumlins.
  • Stratified diamicts include the presence of sand and silt laminations and vary in matrix texture and composition. These diamicts are of minor significance in drumlins and usually occur as lensate beds less than 1m in thickness within the massive units.
• Sand and Gravel Facies: These facies are predominant in stratified drumlin cores, displays fining upwards sequence from coarse gravel to fine and silty clays (Boyce, 1990). Sands in this sequence are moderately to well sorted and planar bedded, further presenting this sequence. Some characteristics of this facies include starved ripple forms, and water escape structures. Starved ripple forms can be found in the fining sediments near the tops of the sequences. Water escape structures and loading of the overlying layers into the sand layers is noted as well as high angle, normal and reverse faulting and shear planes. Soft deformation structures are usually present within the top 1 of this facies. Crudely stratified gravels that are poorly sorted are typical in this facies, occuring both clast and matrix supported, consisting of tabular units (Boyce, 1990).
• Slits and Mud Facies: The silts and mud facies is a minor element in drumlin forming material (Boyce, 1990). It display parallel laminations occurring as thin interbeds, which are associated with the fining upwards portion of the sand facies. An abundance of drumlins to the south of Peterborough encompass stilts and mud facies (Boyce, 1990). Characteristic sedimentary structures of silt and mud facies include water escape structures, slumping and brecciation of rhythmites.

These groups can be further subdivided where distinct sedimentary structures define a separate lithiofacies type (Boyce, 1990).

• Till Cored Drumlins: These drumlins are formed zones of relatively high basal shear stress, in which the bedrock surface has been partially exposed beneath a thin till layer (Boyce, 1990). This formation has been said to be related to the occurrence of northeast facing scarps in the bedrock surface. With reducing bedrock relief, drumlins will become more elongated and eventually result in spindle formation to flute and eventually glacial pavement mantled by a thin drift.
• Stratified Core Drumlins: drumlins with a stratified core were found in areas underlain by substantial thickness of preexisting sediments (Boyce, 1990). This resulted in basal shear stresses that were insufficient to mobilize the entire bed. Thus, it is proposed that they were carved by an erosive deforming till layer, which was preferentially streaming within topographic lows in the substrate. Thus, sediment bodies/mounds would be less streamlined if they existed transverse to flow direction as well as existed under the ice for a shorter period of time (Boyce, 1990).

SIGNIFICANCE OF STUDY

The presence of sorted horizons within the diamicts is incompatible with the deformation hypothesis proposed by Boulton and Hindmarsh (1987). Conversely, they serve to demonstrate that deposition and modification of the sediments by fluvial processes may occur in association with deformation (Boyce, 1990).

Conclusion:

The Peterborough field can be divided into 2 zones, the first being the northern third of the field which is dominated by highly streamlined constructional till-cored drumlins. The second, southward zone transitions to less streamlined, erosional drumlins carved from preexisting sedeiment (Boyce, 1990). The meltwater hypothesis fails to account for the stratigraphic relationships and facies associations within the drumlins. The sedimentology of the drumlins is more consistent with overriding, erosion and distribution of the preexisting sediments as a result of ice moving on a deformable bed (Boyce, 1990). This study’s importance is the combination of sedimentological as well as outcrop and subsurface data in the study of drumlins. The study emphasizes the importance of analyzing a drumlin field with respect to stratigraphy and glacial history, rather than individual drumlins (Boyce, 1990).

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