RE: Adsactly Education - The Mississippi River-The Race West
It seems to me that during most of the conquest of the west the boundary between pioneer settlements and Indian tribes is not precisely delineated. If the Mississippi marks a time this limit, thereafter it is moving. California, for example, is populated very early, even more so with the Gold Rush, while the Great Plains are still almost empty and populated by Indians, who let most convoys pass. Difficult under these conditions to define a precise limit. On the other hand one can probably mark its final progress by the official creation of the states, the arrival of the railway, or the treaties with Indian tribes. It would be interesting, in relation to your chronological limit, to have a map of the American states on the eve of the Civil War. (I imagine you have the right to insert a card.) In any case there is a question: "how is the border progressing?" Progression first diffuse, then more massive, then finally official.
Interesting that you mention California. They were granted statehood in 1849 because of the huge pile of gold that was found there. In the next 50 years they exterminated almost all the native tribes (certainly all on the coast) and made being Chinese illegal, so the ones that stayed were under penalty of death.
The tribes, almost all the tribes, made good faith agreements with the US Government. The government broke every one. The tribes were pursued relentlessly by the US Cavalry.
I do have access to some maps like you suggest. Most 1860 maps show vague 'tribal areas' and areas that are clearly settled or about to be settled. The treaty of Laramie in 1857 granted the Sioux and affilliated tribes the rights to the Black Hills and the area north of the Powder River to the Canadian border. The discovery of gold in the Black Hills changed all that. The broken treaty of Laramie was re written 3 times, the last time it was essentially a surrender document.