Glacial potholes, meltwater channels, salt weathering tafoni and exfoliation on the granite surface in Zhaomiao, Chifeng, Inner Mongolia

A granite hill, about 20 km² in area, is situated 20 km to the south of Lindong Town in the Bairin Left Banner, Chifeng, Inner Mongolia, China.The Yanshannian coarse granite is mainly composed of K-feldspar and quartz.The granite intrusive body has been cut by two sets of vertical joints and a set of exfoliation joints that undulate with topographic relief.The granite hill is about 300~400 m higher than the surrounding area.We investigated the following three mini-landforms, potholes, meltwater channels and salt weathering tafoni and analyzed their formation mechanisms.We also examined the exfoliation and its origin.Potholes on the granite ridge in this area had been reported as wind-erosion evidence by some researchers 20 years ago, or interpreted as a result of chemical weathering lately.However, they were not formed by wind or weathering, but by the fast erosion of melt water from the surface of ice sheet above.According to our explanation, there was a vast icesheet over the Greater Khingan Mountains and the vicinity area in the northeast of Asia in the Late Pleistocene.The thick icesheet tended to create fractures over the granite ridges because of its gravity and drained the melt water through these fractures and thus eroded the bedrock to form potholes on the ridges.Apart from forming potholes on the granite ridges, melt water mainly formed meltwater channels on the surface of the hillside.Meltwater channels are rows of closely spaced erosional notches or grooves on the hillside surface, extending from hilltop to the piedmont, scoured by high-pressure melt water between granite surface and the overlying icesheet.There are abundant tafoni, large and small, along the gently undulating exfoliation joints in this area.These tafoni had been treated as a result of wind erosion and called "wind-eroded habitacles" in the past decades.However, these concave structures are not wind-erosion landforms but typical salt weathering tafoni.Salt weathering is the granular fragmentation of rocks affected by the salt crystallization near/on the rock surface.Salt weathering pits are first initiated along the exfoliation joints and enlarged gradually year by year to form large tafoni.Exfoliation is a common mega-structure in Zhaomiao granite hills.The exfoliation joints are almost parallel to the ground surface, splitting the granite into curved plates.The origin of exfoliation has not been clear until recent years.The sheet joints can be treated as isothermal surfaces underground because of temperature changes induced by Quaternary glaciation.Thus the exfoliation joints reveal the history of Quaternary continental glaciation in the north of Chifeng and the adjacent area.In research area, the Zhaomiao granite hills, potholes and meltwater channels were formed during the later LGM (Last Glacial Maximum) more than 10 thousand years ago, and the landform framework had already been formed by the end of Pleistocene glaciation in the eastern China.Since then, salt weathering has been the major agent to shape the mini-landforms on the flanks of the granite hills.Wind erosion and rain scouring can also be some minor factors but negligible compared to salt weathering.