What are dallol and how were they formed in Niger?

In Niger, dallol are fossilized valleys that once served as tributaries to the Niger River in ancient times.1

These features are remnants of an older hydrographic system that existed thousands of years ago, when the region supported a more diverse environment for agriculturalists, fishermen, and pastoralists. Today, these valleys generally run from north to south and act as zones of dampness, though only a few still contain active water flowing toward the Niger River. Some of the best-known examples of these ancient watercourses include the Bosso, the Foga, and the Maouri wadis.2