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geologyUnderneath the grassland landscape is an ancient backbone of rocks that tell a story involving two hundred million years of deposition under the sea, moving continents, and volcanic activity. The rocky surface has been eroded, smoothed and sculpted by wind, water, and particularly by ice. The result is a complex system of mountain ranges, plateaus and large valleys that lie in a generally northwest to southeast direction.

Over the last million years British Columbia has been covered many times with ice sheets. The last major ice sheets and covered the ancient land surface about 14,000 years ago and were up to 1,000 metres thick. They have defined the grassland landscapes we see today.

As temperatures started to warm up (about 12,000 years ago) the ice gradually melted, yet the process did not occur the same way across the province. In some areas the ice became stagnant, developing ice plugs and forming large, post-glacial lakes. In other areas, such as the Rocky Mountain Trench, large valley glaciers continued to move. It is hard to imagine the huge quantities of water that washed over, under and through the ice, moving boulders, cobbles and gravel and depositing them under and beyond the edge of the ice. The ancient land surface was transformed forever.

By about 10,000 years ago most of the ice was gone from the middle and lower elevations of the interior of the province, leaving a rolling plateau landscape with hills, rocky outcrops, deeply incised valleys, and lakes. Differences in the thickness, form and texture of the glacial deposits left on the landscape influence the complexity and variety of plant communities found in our grasslands.

Glacial till, left behind by the ice blankets much of the interior of BC and is the base in which many of BC's grassland soils have developed . Till contains all particle sizes from clays to boulders, and any size in between.

Kettle lakes developed as blocks of ice left in depressions on the glacial till surface melted on site, leaving many lakes, ponds, and wetlands. Eskers are another relic of times pages. These are long, winding ridges of cobbles and gravels were deposited by rivers that ran under the ice.

Material that was carried by water and deposited, such as that found in eskers, is called glacial-fluvial material. The cobbles and gravels are generally rounded as a result of rolling together along the riverbed.

Colluvium is material made up of loose deposits of coarse angular fragments of bedrock. Colluvial material is moved down slope by gravity. If colluvial material ends up on a glacier, it is then called glacial till.

Aeolian deposits form a layer of silt over many parts of the grasslands. The silt was picked up by the wind from exposed glacial lake beds after the ice left and before vegetation became established.

As the ice melted, water was trapped in the valleys by dams of ice that blocked its flow downstream. Water flowing into the lakes carried fine silts and clays, and deposited them in distinct annual layers on the lake bottom. The clays stettled in the winter when the lakes were frozen and the water was calm. The silts settled in the summer. The slit and clay layers are called varvew and represent one year's deposition. In time the ice dam broke, allowing the water to flow down the valley and draining the lake, often in a very short period of time.

The silt and clay deposits were cut and carved by the flowing water leaving the very distinctive silt terraces and cliffs we see today along many parts of the Fraser River valley, in the Thompson valley east of Kamloops, in the Okanagan valley around Summerland and Penticton, and in south end of the Kootenay valley. Look for them also in many of the smaller valleys that flow into the larger ones.

Want to find out more about the soils of the grasslands?
Go to Grassland Ecosystems and click on Biotic Components.

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