Layers in the rocks hold secrets of Rocky Mountain formation first told 190 million years ago
by Ward Cameron
Have you ever noticed the way a good covering of snow helps to bring out the layers in the surrounding peaks? Later this summer, the mountains will look like one large rock, however with a spring snowfall, they seem to make the individual layers stand out against the sky.
As we look up at mountains like Rundle and see the steeply angled layers, we often forget that these rocks were once under water. I am often amazed to discover a fossil brachiopod, a small clam-like animal, at elevations exceeding 3,000 metres - especially when I realize they've been dead for millions of years. If you were to visit this area a few years ago - say 190 million or so - it would look a little different than today. You would be floating in the middle of a large inland sea. Various materials slowly sank to the bottom and under the pressure of overlying sediments were compressed into the layered rocks we see around us.
It's hard to believe that Mount Rundle, shown here with the town of Banff in the foreground, was once hidden beneath a vast, inland sea.
The steep layers around Canmore, are distinctive of the front ranges of the Rockies and begin with Mount Yamnuska. This mountain, located about 20 minutes east of Canmore, shows the McConnell Thrust fault, and marks the official start of the front ranges. The actual fault line lies right at the base of the large cliff. The front ranges were formed when huge amounts of rock were pushed inland during the second of two periods of mountain building. The first, occurring around 175 million years ago was called the Columbia Orogeny (Orogeny simply describes a period of mountain building) and the second around 120 million years ago and was called the Laramide Orogeny.
back to top
|
Castle Mountain, near Lake Louise, is a landmark for travellers, a beautiful hiking destination, and an excellent example of the Rockies' mid-range castellate mountain formation.
The front ranges are often distinguished by the steeply dipping layers and intense folding visible in its rocks. When the Laramide Orogeny occurred, the rocks forming these mountains were violently pushed inward from the west. This resulted in the rock being thrust over top of the existing rocks of the area. Some of the rocks, like those visible in Mount Rundle, were pushed straight up into the air. There were also much stronger compressive forces which resulted in the intense folding visible in these mountains.
The main ranges are somewhat older and higher. They were formed during the Columbia Orogeny and exhibit a much different character. Unlike the violent compression that formed the front ranges, the main ranges were pushed further and at a much shallower angle. In addition, the layers held together quite well. The mountains of the main ranges tend to appear with layers still lying almost horizontal, but many thousands of feet above the valley floor.
Castle Mountain, about halfway between Banff and Lake Louise, is perhaps the most visible example of a main range peak. Getting its name from its castle-like appearance, it's an example of a type of mountains known as castellate. This form is quite common in the main ranges.
As we look up at the surrounding peaks, we can make out some of the features that distinguish the various ranges of the Rockies. Each was formed slightly differently and has resulted in an almost endless variety of shapes and sizes.
Ward Cameron is a Canmore-based naturalist, photographer and author. His Web site, www.mountainnature.com, is an on-line guide to the nature of the Rocky Mountains. His popular guide book, Canmore & Kananaskis: Best Hikes, Best Activities, includes hiking trails, popular attractions, historical vignettes and superb colour photography.
|
)
)
