It is more common in the forested depression areas. When you are past the woodland on the left, you'll see a large depression. The depression on the left was a lake as late as 1960. These large depressions are called kettle lakes. The organic rich soil is very productive as long as drainage is maintained.
Drainage usually involves using a bulldozer to open a large area in the surrounding hill.
The ditching machine sinks its cutting wheel deep into the soil as it passes through the opened hill, and a tile is installed. Then the soil is pushed back into the cut in the hill. Many times it costs more taxpayer dollars to drain these areas than the purchase price of the entire area. Later, this whole drainage process is repeated when the drainage system breaks down. These drainage funds could be used more efficiently to repair or replace other broken-down tile.