The coalescence of GaP islands, grown on Si(001), Si(111), Si(110) and Si(113) surfaces by chemical beam epitaxy, has been investigated by high-resolution transmission electron microscopy. Stacking faults and first-order twins are observed within islands before coalescence and result from stacking errors during growth on the smaller P-terminated {111} facets of GaP islands. Upon island coalescence, complex moire fringes are observed contiguous to highly faulted {111} planes within epitaxial layers grown on all four Si substrate orientations and are attributed to multiple twinning. Second-and third-order twins are also observed within (111) and (110) layers and their formation is attributed to successive twinning on differently inclined {111} facets. Amongst the four orientations, coalesced growths on the Si(111) surface are the most defective and this may be caused by a higher density of P-terminated {111} facets on islands grown on the Si(111) surface.