Tales and Items of Interest

THE BIG HILL

An interesting geological feature rises to our attention when travelling along the St. Peter’s Road in Marshfield. It is located just to the left of the Suffolk Road entrance and at first glance looks like a big, tree covered hill. “The Big Hill”, as it is referred to by local residents, tells a story about the geological history of this area.

Island Naturalist, Francis Bain, mentions this formation in his book, “The Natural History of Prince Edward Island” published in Charlottetown in 1890. An illustration of it, from that publication, is seen below. Bain refers to it as a glacial moraine “formed in the latter days of the Glacial Era, when the great Continental Glacier had been reduced to local glaciers.” The term now used to better describe this formation, after 20th Century examination of the area, is esker.

Eskers

Eskers are narrow winding ridges that are prominent in many areas that were once covered by ice sheets. The ridges form a conspicuous part of the landscape, being more than 5 metres high and extending for many kilometers. Because they are composed of sand and gravel suitable for road and construction purposes, eskers can be examined in detail in many pit exposures.

The exposed pit faces make it clear that the esker sands and gravels were transported by water and not by ice. The sediments are layered, and pebbles of similar size are grouped together. Cross-beds (the remains of ripples) are common in the sands, and are oriented in the same direction throughout each esker, showing that directed flows - similar to modern rivers - were active. These sediments are very different from those carried and laid down by glacial ice, which are mounded masses of jumbled clay, pebbles and boulders, with little sign of layering or sorting. The ridged form of the water-laid gravel and sand is puzzling, as

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river sediment normally lies within channels cut into the ground.

The solution to these unusual features is that the water was flowing not in a channel but within ice tunnels that opened along fissures in the Laurentian Ice Sheet (which was probably more that a kilometer thick). Because of this, the flowing water gradually built up a mound of sediment that was enclosed by the ice walls and eventually filled most of the available tunnel space. The ice had become stagnant by this time, allowing sub-glacial melt water to transport and sort some of the abundant sediment that the ice sheet had brought down. As the ice sheet melted back, the esker sediment would gradually have been exposed, maintaining its raised form. Although esker formation has not been observed fully, part of this process can be observed within tunnels at the fronts of modern ice sheets.

Eskers are present across Canada. Although they contain few materials that can provide an age date, other lines of evidence suggest that they originated about 10, 000 to 15,000 years before the present, when it is known that the Laurentian Ice Sheet was in retreat. Some ice tunnels at the former southern ice margin in Ontario opened under the waters of the Champlain Sea, leaving large spreads of sorted sand and gravel that were quarried in pits near Ottawa. Likewise, excavation has been done in Prince Edward Island at this site in Marshfield.

Submitted by Barbara Morgan and Martin Gibling