Ok, basically, what did the Midwest's landscape look like before farming, and when did large scale farming begin in the Midwest? Also, are there any areas (I'm guessing protected) that can give me an idea of what the landscape was like before farming?
As a follow up question if that's allowed, do we know of any previously-farmed areas in the midwest/great plains that was abandoned and went back to its 'natural' state?
I'm putting natural in quotes because I've heard the landscape was deeply changed by the Native Americans/First Nations even before the Europeans started farming there.
I think your best bet would be to take a look at things like national grasslands and forests. These are areas protected and managed by the U.S. government with the intent of preserving natural habitats. It should be noted that the degree to which these preserved habitats are considered pristine varies. Many feature a very limited number of grazing permits given to local ranchers, and government employees do manage the land (but usually with the intent of promoting the growth of natural species, removal of invasive species, etc.). I know in the case of my native Oklahoma that most national grasslands are areas of land that came under the protection of the government after the events of the Dust Bowl. Their goal was to restore this mismanaged land to its natural habitat, and they've been fairly successful over the years. Below are some examples of protected areas I'm familiar with:
I think it's important to note that the Midwest/Great Plains are actually very diverse in terms of biomes/ecoregions, so the above links are a pretty small representation of the larger pie. I recommend perusing through Google Maps to find national grasslands and forests in your area of interest. Most should have a page on the USDA's website.
Another helpful resource are biome/ecoregion maps usually put out by state departments or universities. Here are a few examples from some of the Great Lakes states circa early settlement:
I know there's a comprehensive U.S. ecoregion map out there, but can't seem to find it at the moment. You'll likely see some variation among these maps, be it the time period which their representing or the depth of their information - so keep that in mind.
Hopefully these links push you in the right direction! Again, I think it's important to stress the diversity of the Midwest and Great Plains. Oklahoma is different from Minnesota, Minnesota is different from Indiana and so forth. Even on a micro level, there's great diversity even within individual states.