1937 O. T. Jones in 93 272 In view of the fact that a thick mass..may have been formed by successive sliding or slumping of sediments, it is proposed to speak of it as a slump series, and where it is reasonably certain that a mass is the result of a single episode, that mass is referred to as a slump sheet. A slump series is or may be..made up of several slump sheets separated by a greater or lesser thickness of normal mudstones.