The Li River and tributaries drain the area from Guilin to Yangshuo, descending from 141 m at Guilin to 103 m at Yangshuo. Alluvium sediments consisting of well sorted gravels covered by silty sand form floodplains and terraces along its route. Yet, it is the 2,600 m of Devonian and Carboniferous limestones and karst terrain within the Guilin Basin, that gives the area a dramatic landscape. Two distinctive types of karst are found, Fengcong, and Fenglin, which have evolved for the past 10-20 million years, within the Cenozoic.
Fengcong karst dominates the course of the Li River and is defined as a group of limestone hills with a common limestone base, with deep depressions or dolines between the peaks, and sometimes described as peak cluster depression karst. Hundreds of caves are present in this terrain, with 23 having passages longer than 1 km alongside the Li River gorge.
Fenglin dominates the area around Yangshuo and south of Guilin and is defined as isolated limestone hills separated by a flat limestone surface generally covered by loose sediments, and sometimes described as a peak forest plain. The best-known fenglin is the tower karst around Yangshuo. These towers consist of strong and massive limestone forming near-vertical sides with base diameters less than 1.5 times their height. The heights of the towers range from 30 to 80 m in the central basin, but can be as high as 300 m near the Fengcong. In fact, Fenglin evolves from Fengcong by slow and continuous tectonic uplift, associated with the Himalayan orogenic zone, and even slower erosion of the towers.