Geological layers of glacial clayey till cover almost half of the utilized groundwater in Denmark. This report shows that more than 3 – 6 m deep root channels are abundant in the glacial till landscape and likely one of the major controls of groundwater flow and pesticide vulnerability in the tills. Most of the root channels were located as rapid flow paths inside deep Ice Age fractures in the tills. The fractures have for decades been recognized as major pesticide flow paths into the groundwater in the tills. However, in the study flow in the fractures was controlled by the root channels, while the fractures themselves were largely closed. DNA sequencing and radiocarbon analyses of root remains in the root channels reveal they originate from the native prehistoric forest that covered Denmark many thousand years ago. Therefore, similar root channels will presumably be widespread in the clayey till landscapes. The possible dominance of flow in the tills by the ancient root channels instead of fractures, have major implications for strategies to protect the underlying groundwater. This is discussed in the report.