The Badlands Wall constantly retreats north as it erodes and washes into the White River Valley below. This intricately carved cliff divides the upper from the lower prairie and extends more than sixty miles. It is the geologic feature around which park boundaries were drawn, and the Loop Road follows it, sometimes dipping to the lower prairie and then climbing back to the rim. Homesteaders called any place where a wagon could be worked up and down the Wall a pass, but taking a team and wagon through one of the steep, uneven passes was difficult and hazardous. In the 1920s, even some of the more successful homesteaders who had cars struggled with the passes, and no one dared tackle Badlands roads in wet weather without a shovel to dig Model-Ts out of ruts and mudholes. Today, thousands of people negotiate the passes daily in pickups and automobiles.