slough vs tortoise shell

slough

noun
  • The skin shed by a snake or other reptile. 

  • A small pond, often alkaline, many but not all formed by glacial potholes. 

  • Dead skin on a sore or ulcer. 

  • A muddy or marshy area. 

  • A type of swamp or shallow lake system, typically formed as or by the backwater of a larger waterway, similar to a bayou with trees. 

  • A state of depression. 

  • A secondary channel of a river delta, usually flushed by the tide. 

verb
  • To shed (skin). 

  • To slide off (like a layer of skin). 

  • To discard. 

  • To commit truancy, be absent from school without permission. 

tortoise shell

noun
  • The shell of a tortoise, sometimes inaccurately the shell of a turtle, or that material. 

How often have the words slough and tortoise shell occurred in a corpus of books? (source: Google Ngram Viewer )