Willow Creek: Hoodoos, Fossil Oysters, and Petrified Tree Stumps
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Hoodoos are strangely shaped pillars of
rock that are produced through erosion by water, wind, and frost.
They are common in Alberta's
badlands, and those at the Hoodoo
Recreational Area are large, accessible, and famous enough to represent
Alberta on a special series of twenty-five coins. Across Willow
Creek from these hoodoos are fossil oyster beds and petrified tree
stumps that reflect the high and low water levels of the Bearpaw Sea,
the last ancient sea that once submerged much of Alberta.
Hoodoos form where there is a hard
"caprock" which shelters the)
softer rocks beneath it from
erosion. As the soft rocks surrounding the protected sediments
erode away, a free-standing pillar is formed. Gradually, however,
rain will undermine the caprock which topples over and exposes the
softer sediments beneath. Without its protective cap, the hoodoo
pillar will rapidly disappear. Hoodoos evolve as fast as one
centimetre per year on some faces, and are very fragile.
Hoodoos at this site show three layers -- base, pillar, and caprock -- that were all deposited during the Upper Cretaceous Period, between 75 and 70 million years ago. The base is red-brown marine shale of the Bearpaw Formation, which was laid down in the inland Bearpaw Sea. The pillar and caprock are sand and clay of the Horseshoe Canyon Formation. They were deposited along the shoreline in deltas and tidal-flats and by rivers that flowed across this area as the sea shallowed. The caprock contains nearly 40 per cent calcite cement and is therefore more resistant to erosion than the pillar. This hoodoo group consists of 8 to 10 columns ranging in height from one to three metres.
Across
Willow Creek from the hoodoos is another section of the Horseshoe Canyon
Formation. A scramble across the creek and a 100-metre climb up
the slope will bring you to these rocks. Exposed here is a
two-metre thick bed of fossilized oyster shells. These shells were
broken by storms and washed on to the shore as the Bearpaw Sea deepened
briefly and spilled over the delta and tidal-flats. Going farther
up the slope, you will see that the oysters are covered by layers of sandstone,
shale, and coal. These represent another lowering of the sea level
and the resulting influx of river sediments and decaying swamp
vegetation that slowly engulfed and buried the oyster shell
fragments. The shale and coal contain large, petrified tree
stumps, some still in the same position that they grew, some 70 million
years ago. Many geologists believe that these trees were drowned
by a quick rise in the sea and were preserved in place.