This area (Pl. 9) has a relatively flat surface but there are areas of
gently rolling hills. The eastern boundary of the Grand Prairie is
marked partly by the Balcones fault zone. North of McLennan County,
however, the Balcones fault zone is not expressed at the surface and in
this area the eastern boundary is defined by the western edge of the
Woodbine exposures. Upper and Lower Cretaceous rocks occur at the
surface and dip to the southeast; many of these rocks contain a large
number of invertebrate fossils.