 This is because the source of the Arkansas, which was believed to be near the 42nd parallel, is actually hundreds of miles south of that latitude, a fact that was not known until John C. Frémont discovered it in the 1840s.The Adams–Onís Treaty thus delineated the southern (Red River) and primary western (100th meridian west) borders of the future state of Oklahoma.It was also by this treaty that the land comprising the Oklahoma Panhandle was separated from the rest of the future state and ceded to the Spanish government.