Drainage basins are the principal hydrologic unit considered in fluvial geomorphology. Research at SANAE IV include invasion biology/ecology, geology, geomorphology and atmospheric sciences. Earth analogue studies are particularly common in planetary geology, geomorphology, and also in atmospheric science. The geology and geomorphology of the islands is varied. The scientific study of landforms is known as geomorphology. Like the present day Geomorphology, Seismology, Topology etc. Still others investigate the relationships between ecology and geomorphology. The vegetation also contributes wood debris to streams, which is important to maintaining geomorphology. Croatia can be subdivided between a number of ecoregions because of its climate and geomorphology. Stacks are formed over time by wind and water, processes of coastal geomorphology. It is a unique type of petrographic geomorphology found in China. Mass wasting and submarine landsliding are also important processes for some aspects of marine geomorphology. The terrain is quite varied and is thus divided into several areas based on geomorphology. He created a theory of land formation involving concepts accepted in modern geomorphology. Numerous systems exist to classify streams according to their geomorphology.