Let me turn that around and ask what part of Texas did you know, and is that the only part you've ever seen? A large section of it is literally called Hill Country. With a range from 8749 feet above sea level down to actual sea level, there is no definition of the word flat that applies to Texas.Where is Spearman that the land is hilly? The Texas I knew is flat.
This looks like ice age glacier-sculpted terrain, like in the upper Midwest. Maybe Michigan or Minnesota or Wisconsin.