The United States Geological Survey provides us with the best maps out there of Montana's undeveloped country. They have been mapping the west since the 1800's, and most people who spend a fair amount of time outdoors have found an appreciation in the utility and accuracy of a 7.5 minute quad. What many don't know though, is that they provide access to all of their historical maps as well.
This treasure trove of maps is a useful and fascinating source of what can only be described as art. Cartography has changed drastically in the age of the computer, and even though we can do things now that past map-makers would have never even imagined, there is part of me that envies the blood, sweat, and tears that was necessary for making maps in the past. Cartographic styles and Toponyms have changed, rivers have migrated, and place names have been forgotten or made more politically correct. Towns have disappeared. Roads have replaced trails.
In Montana, depending on the spatial coverage and scale you are looking for, there are maps available going all the way back to 1885. The really cool thing is the maps are geoPDFs, so you can use them on your GPS enabled smart phone and walk through (or float over), some historical country and reference the past.
You could float your boat over the now inundated river bottoms of the mighty Missouri, in the 1950 "Lake Sewell" map - the precursor to today's Canyon Ferry Lake.
Or visit the Three Forks of the Missouri and the Valley that goes with it. In 1888, 100 years before the scourge of the subdivision swept across the Gallatin Valley.
The "Hebgen Dam" map of 1950, before the earthquake of '59 caused the northern Henry's Lake Range to collapse into the Madison River, killing 28 people and creating a lake that exists to this day.
I know these maps are tough to see, but that is because they have so much detail (and blogspot has an upload size limit), and is all the more reason you should go download those that interest you. Wander and wonder through the geography of yesteryear in places that have evolved on trails that have been plowed under. Every where has been mapped - but not everything. Everything changes perpetually, and it's pretty cool to see where and how.