A recent trip got me thinking. We camped in the very western tip of Maryland and the southwestern part of Pennsylvania, a gorgeous area in the Allegheny mountains. 

Partly because of the sites we visited, and partly because I tend to get white-knuckled while driving in the mountains (whether on 15-mph ess curves or on 70-mph I-68 with locals flying by me), the trip got me thinking about how people in the past have transported themselves through their natural environments.