Geol102 lecture 20 : anticlines and synclines. Folds: rock structures formed by ductile deformation: rocks at crust are brittle and like to break, going deeper they start to act plastic, they bend and fold instead, fold geometry and nomenclature. Parts on either side of axial plane. Downfolded or troughs of rock layers \/ Symmetrical: limbs of fold are mirror images, upright folds with vertical limbs. Asymmetrical: limbs are not identical, overturned (recumbent) One or both limbs are tilted beyond vertical. Past overturned to recumbent = inverted limb. Axial plane almost horizontal: when dipping in same direction = overturned limb plunge. Upwarped circular features: oldest rocks in center, flat lying sediments have been bulged up. Downwarped circular features: youngest rocks in center, flat lying sediments sunk down like bowl, glaciers can weigh down and create a basin, monoclines. Large, steplike folds in otherwise sedimentary strata. Faults and joints: rock structures formed by brittle deformation: faults.