Written by Jason Vanin
The Manitoba Escarpment is a prominent preglacial geographic feature along the Saskatchewan-Manitoba border. Over 675 km in length, the escarpment also marks a prominent elevation change from the Second Prairie Level in Saskatchewan to the low-lying First Prairie Level in Manitoba. It is cross-cut by four glacial valleys named after their respective rivers: Red Deer, Swan, Assiniboine and Valley. These glacial river valleys divide the escarpment into five highlands: Pasquia Hills, Porcupine Hills, Duck Mountain, Riding Mountain and Pembina (Turtle) Mountain.
The majority of the escarpment features are composed of marine sedimentary rocks, a layer approximately 700 metres thick overlaying the Precambrian Canadian Shield basement. These rocks were deposited from the Early Ordovician period 480 million years ago to the Late Cretaceous period 75 million years ago.
During the Cretaceous period, the Western Interior Seaway connected the Arctic Ocean to the Gulf of Mexico over the span of the continent. The types of sedimentary rock in the escarpment's stratigraphic record reflect the oceanic conditions of the time they were deposited. At certain times, the seaway was inhabited by tropical coral reefs resulting in limestone and dolostone units. When oceanic conditions changed, leaving stagnant water or seas unsuitable for coral, units of shale; sandstone; siltstone; and rock salt were left behind.
The Manitoba Escarpment is the eastern erosional edge of the Western Canadian Sedimentary Basin (WCSB). The WCSB is comprised of sediments deposited while most of western Canada was covered under a shallow ocean. This region of the WCSB was uplifted during a later stage of the mountain building events on the western edge of the continent from 84 to 50 million years ago (Bamburak,. et al 2004). The compressive force of mountain building tectonic activity in the west caused the Precambrian Canadian Shield basement to flex, uplifting the eastern edge of the WCSB over 1600 km away which later formed the Manitoba Escarpment.
Click here to view a cross-section of the geological formations that underlay western Canada.
Since the escarpment is an erosinal edge of the WCSB, rocks beneath the surface of the Second Prairie Level are often exposed. The oldest stone outcrops are Cretaceous and unconformably overlayed by glacial sediments deposited under the cover of ice sheets until their retreat 12,000 years ago. The unconformity marks a missing 93 million years in the escarpment's stratigraphic record.
Glacial deposits are till comprised of coarse gravel with many boulder-sized rocks throughout. The till reaches a maximum thickness in the Duck Mountains at over 200 m (Bamburak,. et al 2004).
The present-day escarpment was carved during the post-glacial history of the last 12,000 years. As the glaciers retreated and melted, water accumulated in Lake Agassiz, a glacial lake that once covered most of present-day Manitoba. The high volume of meltwater cut large valleys across the escarpment while draining into Lake Agassiz.
As the meltwater accumulated and the lake expanded, it eroded the escarpment westward, and beach sand was concurrently deposited. The rolling hills and sandy soil found on the eastern edge of the escarpment are the results of the lake's moving shores over the course of its history. After Lake Agassiz drained into the ocean, the escarpment developed its own drainage patterns into Lake Winnipegosis and Lake Winnipeg – remnants of the ancient lake. These smaller rivers eroded valleys and canyons, exposing outcrops of Cretaceous rock.
Bamburak, J.D., Christopher, J.E. 2004. Mesozoic Stratigraphy of the Manitoba Escarpment - WCSB/TGI II Field Trip, Manitoba Geological Survey/Saskatchewan Geological Survey. Winnipeg, Manitoba, September 7 to 10, 2004. https://www.gov.mb.ca/iem/geo/willistontgi/downloads/fieldtripguidebook.pdf