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The red beds in Zhejiang province of China host the highest concentration of Dan-xia arched rock shelters in the world,just as the Colorado Plateau in the western USA hosts the world's largest concentration of natural arches and bridges.This study investigated the geological background of the arched rock shelters and compared them to the natural arches and bridges,based on field study and a literature review.It was found that Zhejiang arched rock shelters differ from Colorado Plateau natural arches and bridges in geometry and for-mation mechanism.Statistical geometric data on arch geometry shows that Danxia arched rock shelters in Zhejiang tend to be relatively flat.They are relatively low features with long spans,and great depth.The natural arches and bridges on the Colorado Plateau are similar to each other,but the bridges are larger than the arches.The geometric differences between the arched landforms could be attributed to their different geologic history and to their differ-ent formation mechanisms.The arched rock shelters in Zhejiang are formed by differential weathering between sandstone and conglomerate due to moisture-induced tensile stresses.In contrast,natural arches on the Colorado Plateau are closely related to the Salt Valley an-ticline,vertical tectonic fractures,and horizontal discontinuities in rock fins.The Colorado Plateau natural bridges were formed by river erosion.