A Rich Geological History
Edmonton sits on strata deposited by various geological processes. One of the oldest layers is likely the massively eroded Precambrian margin of the Canadian Shield.
The next oldest layer is Paleozoic. During this era continental drift took Alberta to a warm, sunny location somewhere between latitudes 30° north and 30° south. The fossils from this time suggest the land was covered by a flourishing tropical sea. Gradually, thick deposits of organic materials accumulated and were buried on the ocean floor. This is the source of our vast oil and gas reserves.
Later, in the Mesozoic Era, the land cooled as continental drift moved Alberta into more northerly latitudes. Lush forests thrived due to abundant rainfall and the fertilizing effect of volcanic ash. Dinosaurs roamed freely. As the North America Plate continued to move north, the seasonal lack of direct sunlight caused the extinction of many animals.
Tropical vegetation gave way to grasses and sedges better suited to a northern environment. Bison, muskox, horses, woolly mammoths and mastodons wandered the grasslands and the thin boreal scrub bush.
Roughly one million years ago, the Arctic ice cap began to grow. Glaciers flowed down to the plains of Western Canada. The last of four major glacial advances of ice buried Edmonton under an ice sheet more than 1 km deep.
The Dramatic Break Up of Lake Edmonton
As the thick ice sheet began to melt, a gigantic ice dam trapped the water. For almost 100 years, “Lake Edmonton” was walled in on three sides by sheets of ice. Water covered the area from Morinville to Leduc, from Stony Plain to Fort Saskatchewan.
When the lake disappeared, it did so in a flash of geological time. Water found a low point under the dam and began to flow south. “The force of the flow ate at the ice above and the ground below, rapidly enlarging this sub glacial drain until, in a torrent of water and ice and rocks and soil, the lake poured out of its icy basin and into the Battle River, carving a deep channel as it went.” (B. Huck and D. Whiteway)
One theory suggests the flood lasted only a few weeks. Imagine huge flows moving several thousand cubic kilos of rock and sediment! The flood set the stage for the appearance of the North Saskatchewan River. Water flowing over the old glacial lakebed eventually established a preferred channel. Erosion then formed the Edmonton River Valley system we see today.
Whitemud Creek & Larch Sanctuary
Whitemud Creek is a tributary of the North Saskatchewan River. The Palliser Expedition named it for the white-coloured mud that settled out when Lake Edmonton covered the area. Below this mix of silt and clay there is glacial till, then the bedrock of the Horseshoe Canyon formation. It is composed of sandstone, clay shale and numerous coal seams.
The topography varies considerably in Larch’s Sanctuary’s stretch of Whitemud Creek. It ranges from relatively flat in the north to steeper slopes in the south along the creek’s banks. In places the slopes are incised with deep gullies due to erosion.
Of special interest in this portion of the Whitemud Creek valley is the elevated oxbow, a remnant of the earlier river that once sat at a higher elevation than the present creek bed.