The study looked at sediment on the Brazilian shelf and found three zones with different types of sand. These zones show where sea levels stayed the same during the past. The outer zone is at 170 meters deep, the middle zone is at 100-110 meters, and the inner zone is closer to the shore. The sand in the Amazon area comes from rivers, while the sand in the south is older and being moved around. The researchers think the sea level was at 170 meters 40,000 years ago and at 110 meters 18,000 years ago. They need more tests to be sure.