At the beginning of the movie, supposedly outside of Dallas, Texas, where the boy and firemen were consumed by the "blob", in numerous shots you can see mountains, or at least large hills on the horizon. As anyone who has ever been to Dallas knows, there is not a mountain to be found for hundreds of miles
Not to mention, Dallas is not in the middle of a desert as depicted in the movie. Also, when they pan up to show the Dallas skyline, they would have to be in the middle of South Dallas - a very old part of Dallas with no new neighborhood developements. When they showed this scene in the Dallas theaters, the entire audience started laughing hysterically!
This is supposed to be way south of Dallas. I live in Dallas, so I know that there are not that many hills, but if they were far enough south, it does start to become hilly. That also relates, because this isn't supposed to be inside the city of Dallas, just in newly constructed suburbs. Last, though Dallas isn't in a desert, in the summer the ground does start to look like that.