The impact of soil erosion and modern technology on North American agriculture can be devastating. The discussion of soil erosion and its effects dates back three hundred years, when a group of Pennsylvania farmers tried to find ways to slow the effects of the changes occurring on their land. In Tennessee, a state once covered with indigenous tall trees, there are large areas that have been washed away of soil. In Arkansas, a river that once flowed freely is now covered with so much topsoil that a person could walk across it. The most dangerous type of soil erosion is sheet erosion, a gradual erosion that accounts for nearly half of the naturally occurring erosion of cultivated land in the United States. Soil erosion has been blamed for the abandonment of the farming town of Goforth, Texas, where jobless migrant field workers were left to forage for food. Many of the nation's one million transients have fallen on hard times as a result, directly or indirectly, of soil erosion and its impact, as well as the advent of modern technology on agriculture employment. Modern technology, which has replaced the Mexican cotton-picker with a machine that can do in twenty minutes what a laborer could accomplish in two days, is equally responsible for displacing many field workers. In Iowa, where horses have been replaced by heavy machinery, cornpickers have been made absolete.