Another way to get these
rock formation is through weathering and
erosion. This would mean
water is the main
agent, in the form of
precipitation and
ice.
The process usually works like this:
Water, usually as ice or snow, builds up on the surface of a rock. During the day, if it gets warm enough or the sun is shining, the water will melt and seep into cracks in the rock. During the night, the temperature drops, causing the water to freeze again and this freezing forces the cracks to get bigger. Eventually, the cracks get so big that the rock falls apart under it's own weight. This will make parts of the rock break away, leaving part of it still standing. What happens with hoodoos is that the rock gets a little softer further from the top, so it wears quicker and the top portion hangs around, eventually the weathering/erosion works down along a tower of rock. Really cool examples of hoodoos can be seen in Bryce Canyon National Park.
Another example of a hoodoo is the man-made version. As you are walking along trails, in Grand Canyon and Zion anyway, you will notice little piles of rocks. Usually flat ones that would be good for skipping along water, piled one on top of another. Theses are also known as hoodoos. When I first saw them, I thought someone was trying make some kind of pagan altar. Shows you where my head is...