Field and petrographic evidence indicates that many bioclastic limestones interbedded within pelagic claystone and marlstone in the Juana Diaz Formation of the middle Tertiary belt of southwestern Puerto Rico are products of transport and deposition from shallow-water into deep-water environments by sediment gravity flows. While the claystones and marlstones contain only planktonic and some deep-water benthonic foraminifers, the bioclastic limestones consist principally of tests and skeletal fragments of organisms indigenous to shallow-water environments. Many of them show graded bedding and some have flute molds. A characteristic feature is the lack of mixing with the underlying pelagic sediments. In one area between Ponce and Juana Diaz along the San Juan-Ponce Expressway, a mass of limestone 115 m wide and 15 m thick is emplaced within the pelagic mudstones of the Juana Diaz Formation. Its entire cross section is exposed and clearly shows that the mass is an undisturbed portion of Lepidocyclina and algal bank that had been lodged within the deep-water sediments by gravity sliding from the reef zone.