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Click on picture for closer look at Table Rock.
Hoodoos
What is a hoodoo?
What is a hoodoo made of?
What are the parts of a hoodoo?
What is erosion?
What are ironstones?
What is a hoodoo?
A hoodoo is a rock formation which is caused by differential resistance to erosion. This means that some parts of the hoodoo are harder than others and these hard parts last longer. In the hoodoos above, the tops are the hardest part and they protect the soft rock below from wearing away.

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What is a hoodoo made of?
The hoodoos in the Milk River valley are made of sandstone which is sand-sized particles cemented together by calcite, silica, or an iron oxide. Some sandstone is quite hard, but sandstone can be so soft that it brushes away when you touch it.

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What are the parts of a hoodoo?
Most of a hoodoo is soft sandstone. In this soft sandstone, there may be layers (formed as sand was laid on an ancient beach and holes (formed as ironstones fell out). On the top of the hoodoo, there is usually a caprock. Caprocks are made up of many small layers and they are very hard. Caprocks protect the soft sandstone underneath from erosion. When there is no caprock, the soft sandstone erodes away very quickly.

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What is erosion?
Erosion is the wearing away of a substance. Wind, rain, and freeze-thaw cycles cause the erosion of hoodoos.
Wind throws particles of sand at a hoodoo, knocking particles off that hoodoo. When it rains, the water washes particles of sand off the surface of hoodoos. Freeze-thaw cycles involve the expansion of water as it freezes. So when water gets into a little crack on the surface then freezes, it expands, causing the crack to get bigger. This allows more erosion to take place. Hoodoos are constantly changing shape, however, it happens so slowly that we usually do not notice it.

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What are ironstones?
Ironstones are reddish-brown rocks that are very hard. When the sand was being deposited on the ancient beach, living organisms became trapped in the sand. This organic material slowly attracts molecules of iron mineral around it. Slowly, over millions and millions of years, layer upon layer of iron, an ironstone concretion is formed. Ironstones are so hard that when they are exposed on a hoodoo, the soft sandstone around them is eroded away and the ironstone falls out, leaving a hole. The holes in the hoodoos can result in very exotic and curious forms.

These sandstone layers have been exposed for over 15 thousand years. During this time, they have been blasted, cracked, and washed away by erosion. For hundred of years, the Blackfoot people visited this valley and believed that this amazing place was sacred and the home of spirits. Although it is rare for rock art to be found on hoodoos, there are both pictographs and petroglyphs upon the sandstone cliffs along the Milk River.
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Hoodoo, Writing-On-Stone Provincial Park
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