The Gambia River is the country's dominant feature, flowing across a plateau of Miocene-Pliocene sandstone, compacted sediment composed predominantly of quartz grains formed from 23.7 to 1.6 million years ago. In the east, narrow valleys are separated by broad interfluves or flattish hills. In the west, lower and smaller sand hills alternate with depressions filled in with sand so that a flat plain is formed. The climate is of the drier tropical type, characterized by a short and intense rainy season occurring between June and October and by a longer dry season. Near the coast the rainy season lasts longer and the rainfall is heavier, diminishing eastward. At Yundum the average annual rainfall is about 51 inches (1,300 millimetres) and the mean monthly temperature is 77° F (25° C), while at Basse Santa Su, about 270 miles inland, the comparable figures are 43 inches and 82° F (28° C). The relative humidity is high but drops from December to April, when the dry northeastern wind known as the harmattan is dominant.