Final answer:
The subject of this question is Glacial history, which refers to the study of past glaciations and their extent. The farthest extent of glaciations in North America is approximated by the boundary between Canada and the United States.
Step-by-step explanation:
The subject of this question is Glacial history. Glacial history refers to the study of past glaciations, including their extent and impact on the landscape. In this case, the question specifically asks about the farthest extent of glaciations in North America, which is represented by the boundary between Canada and the United States.