Publication QSI by Alan V. Morgan
Shortly after the advent of aerial reconnaissance, aviators in Europe noticed that strange markings could be seen beneath fields of ripening cereals in various parts of northern Europe and in many parts of the British Isles. Investigations by archaeologists showed that these markings were the sites of structures constructed by earlier inhabitants of the region. They included circular Bronze Age or Iron Age hut complexes, Roman roads and marching camps, and medieval villages. The markings were produced by moisture variations in the substrate, reflecting the presence of buried walls and ditches which delineated the former settlements. The moisture differences were outlined by differential growth and ripening of crops in fields above the buried structures. Occasionally markings were noted which pre-dated the earliest known human settlements, and which had no obvious anthropogenic origin. By the late 1940s these were recognised as signs of former periglacial (cold, but non-glacial) conditions which had prevailed in the recent geological past, usually in Late Quaternary time. They involved stone stripes and small-scale polygons as well as larger features, known as ice-wedge polygons which, today, can only be found in areas of continuous permafrost in Arctic North America, Siberia, and other areas of extremely cold climate.
Ice-wedge polygon networks are widespread in southwestern Ontario. They reflect much colder conditions which prevailed when the last ice was starting to melt from the region about 15,000 to 13,000 years before present. They can usually only be seen from low-flying aircraft, but also very occasionally at ground level, when the moisture conditions are just right.
Because of modern observations made over the past 30 years in Arctic regions we know how these ice-wedge polygons are formed. The process begins by a sharp temperature drop, perhaps initiated by the passage of a very cold air mass, in the late Fall. The temperature drop is so extreme that the ground contracts and cracks, forming a narrow void, extending for hundreds of metres, and which penetrates the substrate to a depth of a metre or more. During the winter, snow blowing across the tundra surface infills the crack and freezes to form a thin ice vein. The following spring the ice vein thaws, but it refreezes the following winter and the ground preferentially cracks again along the same ice vein. New snow blows into the crack and freezes, gradually "wedging" the ground open. After the passage of several centuries the wedge might have grown to 50 cm or more across and could penetrate the ground to a depth of a metre or more.
The southern Ontario polygons were found in an area extending from near London, north and east to Kitchener-Waterloo and north to Mt. Forest. They are mostly confined to two till units, believed to be approximately contemporaneous, but one being laid down by Lake Huron ice (The Tavistock Till) and the other by Lake Erie/Ontario ice (Port Stanley Till). Both units were deposited approximately 15,000 years ago. The polygons reflect the time frame immediately after this, most likely from 14,000 to 13,000 years ago. At that time a series of large pro-glacial lakes existed in - what are today - the Erie and southern Huron lake basins to the south of the "Central Ontario Island" an ice-free area with retreating ice to the west, north and east.
Most of the ice-wedge polygons were located by an examination of aerial photographs followed by targeted overflights at different times and in different years (Morgan 1972). Ground investigations(Greenhouse and Morgan 1976) showed that the structures could be located by geophysics (electrical resistivity), and they were systematically trenched to provide cross-section profiles.
The presence of ice-wedge polygons following ice retreat helps Quaternary scientists to resolve the climate in the period immediately following deglaciation. The mean annual temperature is estimated at - 3 degrees C to -4 degrees C with a July average temperature (based on fossil insect evidence at 13,700 yr. B.P.) of about 10 degrees C (Morgan 1982). This was the climate experienced by the first large mammals, such as the woolly mammoths (see the Spring 1996 issue of WAT ON EARTH), which arrived in the region perhaps 13,500 to 13,000 years ago.
References
Morgan, A. V., 1972. Late Wisconsinan ice-wedge polygons near Kitchener, Ontario, Canada. Canadian Journal of Earth Sciences, 9 (6), 607- 617.
Morgan, A. V. and Greenhouse, J. G., 1977. Resistivity Mapping of fossil permafrost patterns in southwestern Ontario. Canadian Journal of Earth Sciences. 14 (3), 496- 500.
Morgan, A. V. 1982. Distribution and probable age of relict permafrost features in southwestern Ontario. p.91-100 In: The R. J. E. Brown Memorial Volume. Proceedings of the Fourth Canadian Permafrost Conference. (H. M. French, ed.), Nat. Res. Council of Canada, Ottawa.