大鹏半岛海岸“仙人石”及周边的花岗岩石蛋是大鹏半岛海岸旅游的重点景点之一,一直被认为是典型的海蚀柱.调查发现仙人石所在海岸为现代潮滩,波浪动力极弱,不足以形成海蚀地貌,而且,石蛋的凹槽朝向并不指向大海,其海拔高度变化很大,因而不是在某一海面上形成的,难以用波浪作用来解释其成因.文章根据野外调查的结果认为,“仙人石”和周围石蛋形成是花岗岩化学风化的结果,起源于其所在位置的花岗岩出露后先是经受化学风化而形成突岩地貌,其后进一步的化学风化使突岩崩塌形成了仙人石及散落在周围的石蛋.散落的石蛋和仙人石下部被风化残积物及土壤掩埋.石蛋被埋在地面以下部分因更加潮湿而产生较强的风化作用,并导致石蛋下部形成凹槽.冰后期海平面上升后海水和地表径流冲刷了地表物质和降低原始地面,使凹槽得以出露.仙人石附近的花岗岩深度风化剖面说明大鹏半岛具有形成突岩的物质基础,大鹏半岛中生代燕山期花岗岩入侵之后因新构造运动被抬升出露则是突岩形成的地质背景.%An isolated group of granite boulders with strange-looking sidewalls curving inward on their lower margins is found on the southeast coast of Dapeng Cove,which is a small bay extending westward into the central part of Dapeng Peninsula from Dayawa Bay,Southern China.This group of boulders is one of the most important scenic spots for coastal tourism of Dapeng Peninsula.The biggest boulder(22°33′13.85″N,114°22′58.50″E) is more than 10 m high and is named "Fairy Stone".It has long been considered that the Fairy Stone is a sea stack,and the concave sidewalls of the boulders are marine notches originating from wave erosion.However,the result of our investigations in the field provided lines of evidence that root out the possibility of the wave origin for the boulders:(1) The coast area where the Fairy Stone is located is shielded from direct wave attacks all year round,(2) most of the boulders have the concavities of the sidewalls that face towards the inland region rather than the sea,(3)the height of the concavities had a great variation,indicating that the concavities were not formed at a same horizontal plane or a same sea level,and (4) the tidal flat,which developed immediately next to the Fairy Stone,is the product of tidal action rather than wave attacks indicating that the wave power here is too weak to produce sea stacks or marine notch on the boulders.Linton's two stage surface theory is used to explain the origination of the Fairy Stone and the surrounding boulders:The boulders originated from the decay of a tor which was a residual mass of granite bedrock produced between the land surface and the ground water table by chemical weathering.The tors were exposed by mechanical stripping of the regolith or grus,which were the incoherent production of the chemical action.The further weathering let to the disintegration of the tors leaving boulders scattered on the ground being covered or half covered by the regolith.Because of the moisture difference between the upper and lower parts of the rock-regolith junction,the concave sidewalls,or the flared slopes,of the boulders were first shaped by the chemical weathering below the regolith surface,where there was more humidity.The concave sidewalls were subsequently exposed due to that the regolith was stripped by sea water that rose up to present level about 6000 years ago.The regional geological studies showed that the granite intruded into the region around Dapeng Peninsula during the Late Mesozoic period,and was subsequently uplifted by the Neo tectonic activity.The fact that the exposed granite possesses joints mainly in northwest and northeast directions is the geological background for the two stage formation of the Fairy Stone and the boulders.Core stones surrounded by regolith were found in granite bedrock weathering profiles which were located in the surrounding areas of the Fairy Stone,indicating the possibility that tors were once existed here.The great variance of the concavity height of the boulders denoted the original regolith surface that was in a great variation of height.In contrast,the present land surface was very flat with an elevation similar to that of the local first marine shore platform.It was the evidence that the original regolith which had an undulate surface was stripped out by sea water that was at a same horizontal plane.Karren or flute developed on the sidewall of some boulders indicates that the weathering processes were continuous after the boulder's exposure.
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