I will vote for Heart Mountain in Wyoming:
The reason for it being unusual has to do with its composition and how it was formed.
In most places on earth younger rock strata sits on top of older ones. This order is dramatically reversed on Heart mountain. The upper part of the mountain is 350 million to 500 million year old limestone. The lower part of the mountain is 50 million year old sandstone.
how the mountain came to be there is even more unusual. There are no regional mountain building events that can account for it. 50 million years ago the land surface had been largely flat, sloping very gently to the east, and was at a level about half way up what is now heart mountain.
then a colossal land slide occurred about 49 million years ago, when a huge slab of limestone over 1500 feet thick and over 400 square miles in area, that is as thick as tallest building in NYC is tall and as big as the entire city of New York, detached from highland 20 miles to the west, in what is now Yellowstone national park, and slid 30 miles down a gentle 2 degree slope at maximum speed of 60 miles an hour, breaking up into fragments as it went.
heart mountain is one of those fragments.