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Willow Creek:  Hoodoos, Fossil Oysters, and Petrified Tree Stumps

Hoodoos 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 thePetrified Stump 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.

Reprinted with permission from A Traveller's Guide to Geological Wonders in Alberta by Ron Mussieux and Marilyn Nelson, with permission by the authors and the Provincial Museum of Alberta.

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