This is the point, where the hills don't rise above the plains anymore, but the plains are carved into the space between the hills (ok, on the image it looks just mostly flat anyway)
I thought this was quite an interesting observation. From eastern Texas, the hills start slowly ondulating out of the wetlands. For quite a while, the impression prevails that they grow "organically" out of the land. From here on though, the topography starts to look older, so that the erosion patterns begin to dominate.