Monday the Frank Slide Interpretive Center commemorated the 110th anniversary of that disastrous day in 1903 when 30 million cubic meters (82 million tonnes) of rock swept over part of the mine and town site and travelled an unbelievable distance across the valley. It is a moment in time that lasted just over a minute and left us with the biggest rock festival anyone has ever seen around here. Or was it?
It seems that one Peter B. Jones published a paper in 1993 in the Bulletin of Canadian Petroleum Geology that speaks to an occurrence tens of thousands of years ago north of the Interpretive Center itself that was in the order of ten times that of Frank. Geologists post this on maps these days as the ancient Bluff Mountain slide and it is an interesting story to say the least.
Peter Jones’ evidence notes the unnaturally high topography east of Bluff Mountain and claims it is in fact a large block of Mesozoic and Paleozoic rock that slid down the mountain's east flank. That block, he suggests dammed off Gold Creek to the east of Bluff Mountain and caused the formation of a lake above it which he refers to as Lake Lille. "Lille" was of course the turn of the century coal mining town north of Frank that was built on a remarkably flat meadow above this theoretical slide zone. The paper suggests the damming of Gold Creek allowed Lake Lille to reach an elevation referred to as Lille Pass, a break between Bluff Mountain and Grassy Mountain farther north. This resulted in a water flow westward, not southward, into Blairmore Creek. Ultimately Gold Creek broke through the dam, eroded a course around the toe of the landslide and re-established its southward course. Lake Lille's level then dropped from 110m (360') deep to that of a broad, flat-lying swamp; remnants of which he claims can be seen today close to where it overflowed into Blairmore Creek at Lille Pass. If one was to drill in the right spot up there Jones’ feels one would likely find lacustrine (lake bed) sediments.
While this area is identified occasionally on simplified geological maps as the Bluff Mountain Slide it seems that the majority of landslide experts do not agree with Peter Jones’ theory. This includes another landslide expert, Corey Froese, Manager of the Alberta Geological Survey, who spoke about the Turtle Mountain Monitoring Project at the Interpretive Center ceremony on Monday. According to Corey LiDAR information that covers the Bluff Mountain area does not show the evidence to support Jones’ claim. LiDAR images dispute the evidence of a rock avalanche having moved material from Bluff Mountain into the Gold Creek valley. From the air, bedrock is revealed, and there is no gap in the rock layers to indicate that part of Bluff Mountain has fallen into the valley.
For the uninformed LiDAR (Laser Imaging Detection and Ranging) is a ground mapping system that consists of a laser mounted beneath a helicopter or airplane. It follows a predetermined path over an area and the ground is scanned by means of tens of thousands of pulses per second from the laser. Coordinates and elevations of scanned objects of areas are determined by using extremely accurate differential GPS measurements. This optical remote sensing technology is used throughout the world in areas such as archaeology, geology, meteorology, astronomy and surveying. The results are stunning in their accuracy and clarity.
Moving further south, when Turtle Mountain was flown with LiDAR in 2005 and a digital terrain model created, the most amazing thing showed up. A line of subsidence pits associated with the Frank Mine is clearly discernible in the aerial oblique photo. Not so with a normal air photo in which the subsidence or collapse pits are obscured by vegetation. I was surprised at how high these collapses were up the slope of the failure but in my mind they lend credence to the suggestion that that 5,000 foot long, almost vertical strip of Kootenay Formation coal seam that was being mined then was a factor in the collapse of this unstable anticlinal structure.
There have been catastrophic collapses throughout the world for thousands of years. Many were mapped manually but these days modern scientific methods are bringing to light a clearer picture of occurrences that happened on a scale that is mind boggling. In 2010 two Canadian geologists pieced together a picture of the largest landslide in North America. It was a colossal avalanche of boulders that occurred about 10,000 years ago and permanently shifted the continental divide in an area about 35 kilometers west of Canmore in Assiniboine Provincial Park. The slide is appropriately called “Valley of the Rocks” and researcher’s first calculation of its volume came up with 1,300 million cubic meters of rock (1.3 billion) with the depth of the main slab being over 160 meters (500 feet) in depth. Some piles of rock at the foot of this slide are nearly 300 meters high and that divide shift in the slide area was in the order of 80 meters. Yikes. The sound must have been deafening and the air blast would have flattened a vast stretch of trees ahead of it. Valley of the Rocks was forty three times bigger in volume than the Frank Slide.
But wait. There is an even bigger slide to tell you about. It has to be the grand daddy of them all. It is by far the largest known rock fall ever on Earth- like thirty times bigger than Valley of the Rocks. 50,000 million cubic meters (50billion) in size. It is known as the Saidmareh landslide (sometimes called the Seymareh) and can be found in Southwest Iran. It travelled more than 14 km and had a width of 5 km. It covers 166 square kilometers and some of its limestone boulders can be discerned in Google Earth. Rock avalanches of this type are sometimes referred to as sturzstrom. It blocked two rivers which allowed two lakes to form.
According to Froese remote sensing techniques are being slowly incorporated into geo-engineering practice and here in the Pass airborne LiDAR has proven important in the mapping of coal mine subsidence locations not previously documented.