In mid-western North America,the southern boundary of the north polar ice cap during past ice ages is marked by these two rivers

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The Missouri and Ohio Rivers are the best choice as they are located in the mid-western region of North America and were the southern boundary of the north polar ice cap during past ice ages. The North Platte and Arkansas Rivers are located further west and south respectively, and are not considered to be part of the southern boundary of the ice cap during the last glaciation.

In Mid-Western North America,the Southern Boundary of the North Polar