This can also be caused by damage inflicted on the tree when it was young. I have one on my property that is only about 20-30 years old which some day will look like that. Yes there are many actual trail marker trees still around but it takes some careful investigation to determine if it is an actual Native trail marker. Age of tree, known trails used 150 years ago, pointing towards drinking water or other features. I tell my grand daughters that it is an Indian tree to point towards a tree stand years ago and that is why I place one of my ladder stands in the same area. But you must remember I had them believing at one time I was the one who taught Michael Jackson how to dance.