These photo brings back lots of memories for me. It is a job I did every summer growing up in the Midwest. What is being done is the workers ( usually teenagers ) are looking for rocks in the field. It is usually done in bean and cornfields every spring. When they find a rock they throw it on the wagon. They go up and down the field rows till the entire field is covered section by section. The reason this is being done is that when the combine harvests the crop in fall that a rock does not get into combine and wreck something.