E. H. Davis, Ancient Monuments of the Mississippi Valley
“ Those works which are incontestibly defensive usually occupy strong natural positions; and to understand fully their character, their capability for defence, and the nature of their entrenchments, it is necessary to notice briefly the predominant features of the country in which they occur. The valley of the Mississippi river, from the Alleghanies to the ranges of the Rocky Mountains, is a vast sedimentary basin, and owes its general aspect to the powerful agency of water. Its rivers have worn their valleys deep into a vast original plain ”
