In the book Fundamentals of Geomorphology (Routledge Fundamentals of Physical Geography) by Richard John Huggett there is a sentence " Intruded rocks, which must be mobile but not necessarily molten, may have a direct effect on landforms by causing doming of the surface." What is the difference between molten & mobile rock?
I believe the author's was referring to phenomena like salt diapirs (where mobile salt deposits intrude into heavier overlying sediments). These produce low hills in the gulf coast region of the United States. The salt that is moving is not liquid, it is merely mobile.
I do not believe the author of the book you reference was referring to sand and mud diapirs as these deposits are not considered "rocks".