folds

• An upright fold is when the axial plane is vertical or near vertical 
• Inclined folds are when the axial plane dips between 10-80 degrees 
• Overturned folds are when one or more limbs are the wrong way up. 
• Recumbent folds are when the axis is horizontal 
• Interlimb angles:

  • 120-180 degrees = gentle 
  • 70-120 =  open 
  • 30-70 = closed 
  • <30 = tight 
• Isoclinal folds are when the limbs are parallel to each other. 
• Joining types: 

  • Folding/tension joints are due to tensional forces on rocks due to folding, found in competent rock 
  • Cooling joints are caused by igneous rock cooling and contracting, joints are vertical and perpendicular to cooling surfaces 
  • Unloading joints are formed due to pressure release due to uplift and erosion, joints are parallel to earths surface 
• The presence of jointing and cleavage in rock depends on the type of rock. 
• Competent rocks are brittle and fracture instead of folding, they are more resistant to pressure and develop joints rather than cleavage. 
• Incompetent rocks are weaker and are ductile so they thin or stretch in limbs of folds and thicken in areas like the crest of a fold. Incompetent rocks contain platy minerals and develop cleavage. 
• Slaty cleavage and folds are both produced due to compression at plates like convergent boundaries. Cleavage in this case is referred to as axial planar cleavage, it can develop into shale also. 
• Plunging folds are folds with the axis tilted away from the horizontal. 
• Antiforms plunge towards bed closure. 
• Synforms plunge away from bed closure.
• Stereo nets do not show whether a fold is an antiform or a synform. And they do not show number of folds. 
• Stereo nets do show an upright or inclined fold. And they able you to estimate interlimb angle. 
• Stereonets can show whether folds plunge.