Ellipsoidal Basins

Ellipsoidal Basins is a geographical term used to describe deep, elongated lakes, formed by subglacial activity beneath past ice sheets [1]. Examples of these basins include the Great Lakes and Finger Lakes of North America [1,2]. These basins were formed either where the ice was topographically constrained (Finger Lakes), had vulnerable, softer, geology (the Great Lakes), or in an area of maximum ice discharge (Hudson Bay) [1].

All five of North America’s Great Lakes are pictured in this spectacular image captured by the Copernicus Sentinel-3 mission: Lake Superior, Michigan, Huron, Erie, and Ontario. From: ESA

All these ellipsoidal basins are orientated in the direction of past ice flow and near the ice sheet margin [3]. They have smoothed edges, which is typical of glacial landforms. A more commonly used term to describe this type of lake is glacial overdeepening.

Glacial overdeepenings are large-scale erosional landforms which have been widened and deepened to depths deeper than the pre-glacial valley [4]. Overdeepened basins can be also used to describe other glacial features such as cirques or fjord floors [3,5]. However, here we will be discussing overdeepened basins formed beneath ice sheets (ellipsoidal basins).

Finger Lakes of New York
Satellite Image of the Finger Lakes of New York, an example of ellipsoidal basins. Wiki Commons

Formation of Ellipsoidal Basins

Over the Quaternary Period (the last 2.6 million years), successive cold glacial periods allowed large ice sheets to spread over much of the high latitudes of the Northern Hemisphere, including the Laurentide Ice sheet [4] in North America. The Laurentide Ice Sheet expanded over Canada and some of the United States and was responsible for large glacial landforms seen today in these regions, including ellipsoidal lakes.

Our true understanding of the formation of ellipsoidal basins/ overdeepenings is incomplete because the area beneath active ice sheets is not accessible. However, signs of past glacial activity from glacial landforms within these basins reveals some of the processes involved in their formation [1].

Pre-glacial river valleys act as suitable channels for ice to flow through and become topographically constrained by the valley walls [3,6]. This constraint on the ice allowed overdeepening to occur, concentrating erosion to the base of the ice. The erosional processes acting beneath the ice are typically plucking, quarrying, scouring and abrasion and are all modulated by the thermal regime and the geology [3,6].

Case Study: The Great Lakes of North America

Both The Finger Lakes and the Great Lakes in North America are a classic example of ellipsoidal basins as they are a series of deep, elongated lakes arranged in an hierarchical structure [1]. The Great Lakes are the deepest and largest lakes in North America. Each lake basin originated as a northwardly flowing river, and over successive glaciations during the Quaternary period, was eroded into the present formations.

Satellite image of the Great Lakes, April 24, 2000. From Wikimedia. The Finger Lakes can be seen, just southeast of Lake Ontario.

Click on the map below to take you to an interactive map showing the flowlines of the Laurentide Ice Sheet, the ice sheet extent, and the Great Lakes. Can you also find the Finger Lakes in New York?

ESRI Interactive map showing the reconstructed Laurentide Ice Sheet extent [7], The Great Lakes and Flowlines [8]. Map created by Dr Bethan Davies.
  • The main layers in the map are the ice sheet margins, the directions of ice flow, and the Great Lakes.
  • Notice how the lakes extend in the direction of the ice flow, and are positioned near the ice sheet margin. Both of these characteristics are typical of glacial overdeepenings [3].
  • Try and locate the Finger Lakes in New York. What do they suggest about the direction of ice flow? Has it changed?
  • What do you notice about the margins of the Great Lakes? How do they compare to lakes in non-glaciated areas? Can you find an example of a non-glaciated lake (south of the limits of the Laurentide Ice Sheet) in North America for comparison – how are the margins and overall morphology different?

Case study: The Finger Lakes of North America

Shortly after 14,500 years ago, during the deglaciation of North America and the retreat of the Laurentide Ice Sheet, the 80 km wide Seneca-Cayuga paleo ice stream occupied the overdeepened New York State Finger Lake basins [9]. This ice stream formed mega-scale glacial lineations and other streamlined landforms.

Sookhan et al. 2021 (Figure 1). Lidar topography of f Finger Lakes region of Upper New York State, USA and contiguous parts of Ontario north of Lake Ontario.

The maps showed clear ice flow patters, with landforms representing faster flow (Mega Scale Glacial Lineations) to steady state flow (drumlins), and highlighted the role that the Finger Lakes had on the ice flow speed. Evidence also suggests that the fast ice flow resulted in the increase in erosion potential from abrasion and subglacial meltwater resulting in the continuous overdeepening of the Finger Lakes, causing them to have their deep, elongated structure [9].

Explore the Finger Lakes of New York in the interactive embedded Google Map below.

The Finger Lakes are around 200 m deep, and are cut into the Allegheny Plateau. They occupy narrow, steep-sided bedrock basins, extending up to 70 km into the plateau. The largest Finger Lakes radiate outward to the south, like fingers on an outstretched hand [9]. The bedrock floors of the largest lakes are over-deepened, to well below sea level (e.g. the Rochester Basin at – 244 m below mean sea level). This over deepening indicates that these lakes have been a locus for enhanced glacial erosion over multiple glacial cycles [9].

Drumlins and mega-scale glacial lineations are well aligned with the lakes, indicating the direction of ice flow.

Sookhan et al., 2021 (Figure 5). Geomorphological map of the study region.

We can use the orientation of these geomorphological features and the lakes, and the position of the moraines, to reconstruct the former ice-flow pathways of the Seneca-Cayuga paleo ice stream [9].

Sookhan et al., 2021, Figure 10. Map showing the footprints of the different paths of the palaeo ice stream.

Further reading

Earth from Space: the Great Lakes

Dig the Dunes: glaciation and the formation of Lake Michigan

Origins of the Great Lakes

Life in the Finger Lakes

Open access images used were available under a Creative Commons License.

References

1. White, W.A. (1972) Deep erosion by continental ice sheets. Geoological Society of North America. 83. 1037-1056.

2. Sugden, D.E. (1972) A case against deep erosion of shields by ice sheets. Geology. 4. 580-582

3. Patton, H., Swift, D.A., Clark, C.D., Livingstone, S.J., Cook, S.J. (2016) Distribution and characteristics of overdeepenings beneath the Greenland and Antarctic ice sheets: Implications for overdeepening origin and evolution. QSR. 148. 128-145.

4. Cook, S., and Swift, D.A. (2012) Subglacial basins: Their origin and importance in glacial systems and landscapes. Earth Science Reviews. 115. 332-372.

5. Burschil, T., Tanner, D.C., Reitner, J.M., Buness, H., and Gabriel, G. (2019) Unravelling the shape and stratigraphy of a glacially-overdeepened valley with reflection seismic: the Leinz Basin (Austria). Swiss Journal of Geosciences. 112. 341-355

6. Benn, D.I., and Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder-Arnold, London

7, Dalton, A.S. et al. (2020) An updated radiocarbon-based ice margin chronology for the last deglaciation of the North American Ice Sheet Complex. Quaternary Science Reviews. 234. 106223.

8. Marigold, M., Stokes, C.R., Clark, C.D., and Kleman, J. (2015) Ice streams in the Laurentide Ice Sheet: a mew mapping inventory. Journal of Maps. 11(3). 380-395.

9. Sookhan, S., Eyles, N., Bukhari, S. & Paulen, R. C. LiDAR-based quantitative assessment of drumlin to mega-scale glacial lineation continuums and flow of the paleo Seneca-Cayuga paleo-ice stream. Quat. Sci. Rev. 263, 107003 (2021). https://www.sciencedirect.com/science/article/pii/S0277379121002109

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