Trees on the side directly exposed to sunlight generally grow faster than on the opposite side,a phenomenon termed plant phototropism.There are in situ vertical trunks of silicified wood in the Xiadelongwan area of Yanqing County,north Beijing,where the first National Geologic Park of Petrified Wood of China has been built since 2002.A few trunks have well-preserved growth rings.One petrified stump from the formation shows a positive phototropism direction of SW23O0.As compared with the modern normal growth stumps in Beijing plain area,which have a positive phototropism direction of SW210 °± 5°,the evidence of wood phototropism supports the conclusion of previous palaeomagnetic studies that the North China Plate has rotated clockwise since the Late Jurassic.The known petrified wood stumps in the Yanshan-Liaoning area are mainly found from the strata of 165-136 Ma,which corresponds to the main stage of the Yanshanian Movement.
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