Above, all roads in California. Found at reddit/r/mapporn of course.
Roads are a good first approximation for population density, which drops off in mountainous country of course. And you can compare to Pennsylvania, where you can really see the drop-off north of I-80, which is the less rugged but still foreboding Allegheny Plateau. (Contains the highest point on I-80 east of the Mississippi; somehow the mountains in Pennsylvania are just scary, captured well by Kerouac in On the Road.*) This part of the state wasn't really explored until the 1790s, after the Revolutionary War. Wiki map below.
Above: the Grand Canyon of Pennsylvania, Northern PA, just because. From Geo Times.
*"We walked seven miles along the mournful Susquehanna. It is a terrifying river. It has bushy cliffs on both sides that lean like hairy ghosts over the unknown waters. Inky night covers all. Sometimes from the railyards across the river rises a great red locomotive flare that illuminates the horrid cliffs."