Short answers, pulled from the story.
A fold is defined as a stack of originally planar surfaces that have been bent or curved during permanent deformation. This process transforms flat sheets deposited on the floor of a deep sea basin into complex curves found in limestone and chert layers. The scale of these structures varies from microscopic crinkles to mountain-sized folds that define entire ranges.
Fold tightness is defined by the size of the interlimb angle measured tangential to the folded surface at the inflection line of each limb. Gentle folds possess an interlimb angle between 180 degrees and 120 degrees, while tight folds range from 30 degrees to 0 degrees. Isoclines or isoclinal folds have an interlimb angle between 10 degrees and zero, resulting in limbs that are essentially parallel.
Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata to accommodate volume preservation. Buckling occurs by the simple buckling of a planar surface and its confining volume where volume change is accommodated by layer parallel shortening. If folding deformation cannot be accommodated by these methods, rocks are removed from the path of stress through pressure dissolution.
Folds create voids between layers where lower water pressure triggers the deposition of minerals to form veins. These concentrated sites of trace minerals gathered over millions of years become the target of the mining industry. Highly folded rock is therefore a key indicator for locating valuable mineral veins.
Concentric folds maintain uniform layer thickness and are caused by warping from active buckling of the layers. Similar folds display thinning of the limbs and thickening of the hinge zone. Ramsay proposed a classification scheme for these folds based on the curvature of the inner and outer lines and the behavior of dip isogons.