2. Sea Floors, Volcanoes & Ice Sheets – Natural History, 125 million BP – 12,000 BP

Surface Landscape – Remnants from the Pleistocene

Profile through southern Manitoba along a line just north of the international boundary showing relief, natural vegetation and surface materials.

The Boundary Trail National Heritage Region possesses an unusually wide range of interesting and unusual landscape features. There are super-flat flood plains; a one hundred meter high linear escarpment; deeply eroded flat-bottom river valleys; v-shaped gorges; rolling hills; large isolated buttes; and densely forested rolling highlands permeated with small sized lakes – to list but a few of the many interesting natural features to be found in the region.

And all of it – the entire current topography of the current BTNHR, was created during the melting of the last continental ice sheet which began to melt away approximately 15,000 years ago. At one point the region was buried under ice sheets 2,100 meters thick. 

Map showing the cumulative extent of the waters of glacial Lake Agassiz.

By 10,000 BP the plateau, now known as Turtle Mountain, was the first area in what is now Manitoba to be ice free and before long the region was supporting evergreen forest and various types of forest wildlife. Only a short distance to the north and east, massive amounts of sediment laden meltwater was accumulating along the ice front, creating deep lakes and at times massive spillways a kilometer or more wide draining these shifting glacial lakes. The largest and best known of these glacial melt-water lakes, Glacial Lake Agassiz, covered much of what is currently Manitoba, North Dakota and Minnesota, the former lake-bed of which is now known as the Red River Valley. 

Elevation map of southern Manitoba showing the location of the BTNHR and its major land-form features.

In the areas where the melt-waters did not pond but drained away from the ice face, the material that had been caught up in the ice was simply dropped in place creating a rolling topography of mixed glacial materials, rock, boulders, clays and gravels. In the areas where streams were formed the material they carried were frequently deposited as sand bars, gravel ridges – and similar ‘stratified’ or ‘sorted’ glacial deposits. In areas where lakes, large and small were formed, clay deposits were formed as the smallest of the waterborne glacial materials, settled to bottom in the undisturbed deepest parts of these lakes. The ice sheets did not melt back uniformly. On occasion the ice advanced for some years, and on occasion the rate of ice advance was roughly the same as the rate of melting, In these cases the material carried along within the glacier, accumulated at the ice front creating larger and higher deposits of mixed glacial material. The Cypress Hills, just to the north of the BTHR, were formed in this manner.

Glacial lake Souris and glacial river Pembina, circa 10,000 BP.

Knowing how the various land-forms and topographic features in the BTNHR were formed can make even seemingly minor features of interest and the BTNHR offers some of the finest collection post glacial landscapes on the Canadian prairies. These features include entire districts as well as individual sites.

These include the following:1. erosion feature & stagnant ice moraine (Turtle Mountain); 2. Lake Souris glacial meltwater spillway (Souris and Pembina River valleys); 3. Pembina Hills – Manitoba Escarpement (glacial erosional feature); 4. Glacial Lake Agassiz lakebed – (Red River Valley); 5. Captured Stream – Souris River elbow; 6. Terminal Moraine – Tiger Hills


1.3 Turtle Mountain highlands

View from a municipal road on the northern slopes of Turtle Mountain. 2008.

The Landscape feature known as Turtle Mountain has long been noted and referred to in the diaries and maps of early explorers as a major landmark and important feature on the landscape of the northern Great Plains. It rises from a smooth plain about 1600 feet above seas level to a elevation, at its highest point of about 2400 feet above sea level, or approximately 750 meters above the surrounding plain.

The topography of Turtle Mountain is due in part to erosion and in part to glacial deposition. Approximately two kilometers of sedimentary rock overlays the Precambrian basement beneath Turtle Mountain. This overlay consists of layers of sand, silt and seams of lignite coal. Erosion over millions and millions of years reduced the height of the Mountain before glacial ice covered it.

As the ice thinned and melted with increasingly temperatures, debris that was carried by the glaciers was deposited over Turtle Mountain. These deposits measure about 150 meters in thickness and site atop older deposits and the eroded base. By about 12,800 years ago Turtle Mountain was free of ice. As the glacier melted and the ice front retreated the slumping glacial materials left many shallow lakes and wetlands on the surface of the Mountain and shaped the gracefully rolling hills and dales. Over eons steep sided ravines were cut into the hillsides as rain and runoff waters from the higher, forested elevations to the surrounding plains. 

View of one of the many picturesque valleys to be found along the northern slopes of the Turtle Mountain highlands.

Credible evidence suggests human occupation or use of the Turtle Mountain area that dates back 10,000 years ago, at a time when glacial ice still occupied much of what is now central and northern Manitoba. First known human residents were the mound builders, who left little evidence of their passing other than the mounds themselves. Then the Clovis people who hunted the prairie plains for the mammoth to be followed by nomadic First Nation hunting groups which subsisted off the migrating herds of bison.

Early explorers and fur traders who visited the Turtle Mountain district included La Verendrye during the 1730’s who referred to the mountain as the “Blue Jewel of the Prairie”. During the 1860s the Dakoka moved into the area from the current states of Minnesota and North Dakota to escape conflict with the american cavalry and made their home in Turtle Mountain. The Red River Métis frequently hunted on the plains north of the mountain and wintered in its upland forests from the 1830s to the 1870s. European settlement of the area exploded during the late 1870s and early 1880s.

A forest fire swept through the area in 1898 leaving only the island on Max Lake unscathed, while the rest of the mountain was lest a wasteland of charred scrub. Earle Currie the son of an early area homesteader recalled the event: In 1898 when I was a boy on the farm, the Turtle Mountain forest caught fire and as there was no fire-fighting equipment, it burned for days, the black smoke blocking out the sun. (MHS, Earle M. Currie interview.)

Today the forest and bush land of Turtle Mountain offers a safe refuge for a vibrant ecosystem made up of a variety of plants and animals, birds and insects. As well, Turtle Mountain Provincial Park and the International Peace Gardens provide public access to much of the Turtle Mountain highland areas.

(Sources include: 1. Gerhard Ens, 1982; 2. Norman Wright, 1949, 2. Turtle Mountain Conservation District literature.)

SLIDESHOW: Turtle Mountain – maps and views.


Bentonite Deposits and Marine Fossils

Towards the end of the Cretaceous Period large portions of what is now southern Manitoba was covered by a huge shallow inland sea, known as the Western Interior Seaway. Sometime during this era, about 83 million years ago, intense volcanic activity thought to have originated in the vicinity of present-day Montana, blasted millions of tons of volcanic ash into the skies over the North American continent. Drifting northward, much of this ash eventually settled onto the surface of the Mesozoic seas. Here it sank to the bottom, where it was eventually was covered with other layers of marine deposits before another series of eruptions laid down more strata of ash.
Today this volcanic ash, in layers usually about six inches to a foot (15-30 cm) thick and layered between beds of shale, is a commercially valuable mineral known as bentonite. A creamy-white coloured ‘slippery clay’, bentonite is is formed by the alteration of volcanic ash by seawater and pressure. It possesses a high calcium content and is widely used in detergents, cattle feed and especially in the purification process of mineral and vegetable oils.
Commercial bentonite mining began in the 1930s. When the shale overburden was stripped away to access the bentonite deposits along the Pembina Escarpment, particularly in the Miami area, the skeletons of the dwellers of the Western Interior Seaway seas were found remarkably preserved. These include marine reptiles, (including massive turtles) ancient sharks and even the occasional bird. In some instances, because of the glacial erosion of later overlying deposits, these fossils can be found a little as five feet below the surface.

Some of these remarkable finds are on display in the Canadian Fossil Discovery Centre in Morden, where they form the largest collection of its kind in North America. Among the most fascinating fossils of this collection are the reptile remains, in particular the moasaurs and plesiosaurs.
(Sources: Carr, Karen. C.F.D.C. Pamphlet, 2013. R.M. of Louise History , 1979; BTNHR research files)

Above: Western Interior Seaway Diorama. An Artist’s impression of marine life during the Cretaceous period, evidence of which has been unearthed in the current Boundary Trail Heritage Region. (source: Carr, Karen. C.F.D.C. pamphlet, 2013.)

Left: Map. Western Interior Seaway. As depicted on this map, during the Cretaceous Period much of the interior region of the current North American con-tinent was covered by shallow saltwater seaways brimming with ancient marine life. (Source: C.F.D.C. Pamphlet, 2013.)

Right: Excavation face showing bentonite layers in a pit in the Miami area of the BTHR. (Source: Louise RM local history.)

Above: Canadian Fossil Discovery Centre staff member Cheryl Link provides size context with the museum’s star attraction, “Bruce”, a 13 meter (43 feet) mosasaur skeleton discovered near Thornhill in 1974. (EdLed photo. 2017.)

Above: A diorama display at the Canadian Fossil Discovery Centre in Morden showing bentonite strip mining. (EdLed. photo. 2017.)



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