Finally, during the Younger Dryas stadial, large amounts of loess were deposited in the valley, making it extremely fertile, while the hills around it are composed to this day of sandy loam unsuitable for intensive agriculture, so that the area has been covered with thick forests to this day. This contributed to the isolation of the area, especially considering the extent of the forests in the Middle Ages was much larger than today. The isolation of the valley was only alleviated by the arrival of the railway in 1865 and finally lifted in the interbellum by the construction of a high capacity paved road to nearby Nijmegen.