Cretaceous sandstones span the west and central areas of the republic. The Bakouma Formation includes extensive carbonates, glacial outwash and tillite deposits dated to the Neoproterozoic. Stratigraphically, the lower units of Precambrian rock are granite, amphibolite, gneiss and gneissic-migmatite rock, likely dating to the Neoarchean. These sequences, divided into the Bandas Belt and Dekoa Belt, are often described as a greenstone belt. Granitoid batholiths and dolerite dikes dating to the Proterozoic frequently intrude into these layers. The upper sequence is Neoproterozoic age quartzite and schist and tends to be folded, but only weakly metamorphosed. In the west, the Mambere Formation and the Kombele Formation in the east each record glacial sediments, while erosion and river activity left behind the Cretaceous Mouaka-Ouadda and Carnot-Berberati sandstone formations, as well as the plateaus of the Eocene Bambio Sandstone. During the Quaternary extensive weathering of surface rocks took place, up to 40 meters deep. Iron-rich plinthosols dominate most of the center of the country. Unconsolidated psamment deposits (also known as arenosols) occupy a section of the west and center of the country along with soils grouped under the FAO ferralsol classification of iron and aluminium-rich soils. Hydric, grayish-blue wetland gleysols are found in small locations in the north and along the border with the Congo.