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Strata of sandstone sedimentary rock in a coastal cliff with layers folded down to form a geological structure called a syncline, near Zambujeiro do Mar, Alentejo Littoral, Portugal, southern Europe. A syncline is a trough or fold of stratified rock in which the strata slope upwards from the axis. Folds typically form during crustal deformation as the result of compression that accompanies orogenic mountain building.