What group of people served as the "sturdy base" of the frontier?

Farmers

Explanation:

Farmers were the people who were considered the sturdy base of the frontier. When the territories on the western section of the United States were being settled, many people moved there to try their luck. They hoped that they would be able to acquire land and raise their families in this area. These people were often farmers, and because they were the earliest and most permanent settlers, they were considered to be the "sturdy base" of the frontier.

permafrost layer, it exists beneath a layer of soil, rock or sediment.

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