inverted arch

Webster's Dictionary (GCIDE)

1.Changed to a contrary or counterchanged order; reversed; characterized by inversion. [1913 Webster]

2.(Geol.) Situated apparently in reverse order, as strata when folded back upon themselves by upheaval. [1913 Webster]

Inverted arch (Arch.), an arch placed with crown downward; -- much used in foundations. [1913 Webster]