Source of the Foothills Erractics Train

The search for the source
the Foothills Erratics Train As he systematically mapped the extent of the Foothills Erratics Train during the early 1950s, Dr. Stalker pondered the source of these rocks. He knew from existing geologic maps that similar quartzite existed in the Rocky Mountain Trench near Windermere, B.C., the Waterton Lakes area, the Rocky Mountain Main Ranges in Banff and Jasper National parks, and in the Lake Athabasca area along the edge of the Canadian Shield some 800 to 1300 km to the northeast.

He eliminated sources southwest of the erratics train because his mapping of ice-formed directional features indicated that the ice-age glaciers that had transported the erratics train, had flowed generally north to south, and not south to north. He eliminated the Canadian Shield as a source because the erratics would had to have been plucked from bedrock along the base of several kilometres of glacial ice and transported more than a thousand kilometres beneath the sliding, grinding ice sheet.

Detailed view of pebbly pink to grey quartzite characteristic of the Foothills Erratics. Note the angular shape of the boulder (L.E. Jackson Jr., GSC)

Striations on a quartzite boulder. Its rounded shape and striated surface are typical of boulders tansported along the base of a glacier (L.E. Jackson Jr., GSC)

Boulders transported along the base of glaciers are typically covered with scratches, called striations, or are polished and rounded and streamline in shape. The Foothill erratics almost entirely lack striations. They are also cut by cracks called joints. It was inconceivable to Dr. Stalker that these very large (many exceed 4 m in length) highly jointed, blocks could have remained intact and angular for any distance if they had been carried along the along the base of a glacier. He concluded that they had to have been transported on or near the surface of glacier ice. Only the lower Cambrian quarztite formations to the west and northwest of the erratics train in the Rocky Mountains remained as possible sources, but where? Similar erratics could not be found west of the erratics train in the Bow and North Saskatchewan valleys, and so these were eliminated. Stalker did not have the answer in his 1956 paper, but he was able to describe the physical characteristics of the source area, based upon his conclusions of superglacial transport of the erratics and their characteristics of the layered structure of them refered to as bedding:

"It must have been near a large pass in the mountains to the west that, during one of the glacial stages, contained a valley glacier with a large volume of fairly rapidly flowing ice. A bed of the characteristic pebbly quartzite must have been present high enough on the main or tributary valley walls for the glacier to undermine and quarry it in large blocks and to carry them on its surface. This pebbly conglomerate bed must have been not less than 30 feet thick, the thickness of the Big Rock..."

This is a vivid description of the environment of the lower Cambrian quartzite beds in the mountain faces of the Mt. Edith Cavell-Tonquin Valley area in the headwaters of the Athabasca River, Jasper National Park.

Lower Cambrian (~500 million years) quartzite beds, Tonquin Valley, Jasper National Park (photo by J.O. Wheeler, GSC)

This area was shown to be the source of the Foothills erratics in paper written by Dr. Murray Roed and others in 1967. Although Stalker concluded that the Foothills erratics had been transported by glacial ice originating in the Rocky Mountains, he also concluded that transport by mountain glaciers alone could not explain the great length, narrowness, and changes in elevations which characterize it. He reasoned, on the basis of his regional mapping, that the Laurentide ice sheet must have been present immediately to the east of the Foothills. It prevented the erratics from being deposited in a lobe shape at the mouth of their (then) unknown source valley. The east flowing mountain glacier transporting the erratics joined the south flowing Laurentide ice thus confining it between the western edge of the Laurentide ice sheet and the Foothills. The glacier carrying the erratics remained a comparatively narrow ribbon of ice as it traversed the eastern edge of the Foothills. This explained the narrowness of the erratics train and the small differences between the highest and lowest erratics at any latitude. In Stalker's words: "The erratics from the west would be carried southward near the surface of this combined ice-sheet, and strung-out for a long distance near its margin...". This picture of glaciers flowing out of the Rocky Mountains and joining the south-flowing Laurentide ice sheet has been supported by subsequent mapping along the Foothills by Jackson and others.