I would call them 'gum bends' - they don't appear to have broken the paper fibers. If the bends get so bad that they break the paper fibers, then I call them 'gum creases'. Both are natural and are considered 'faults' although not egregious faults. How do you know if they have broken paper fibers? Dunk the stamp in watermark fluid. Broken fibers will look like dark spots or dark lines in fluid. Gum creases/bends are fairly common for the era.