Rocks were being eroded and little deposition to form Mesozoic sedimentary rocks occurred around El Paso until the middle of the Cretaceous Period, in late Mesozoic time. In the Cretaceous, The El Paso-Ju rez area was near the head of an arm of the Gulf of Mexico called the Chihuahuan Embayment . Shallow water marine sediment was again locally deposited. Today, these sedimentary rocks are visible along the base of the of the western slope of the Franklin Mountain, around the flanks of Cerro de Cristo Rey, and throughout the Sierra de Ju rez. Numerous fossils of marine invertebrates are present, but no remains of terrestrial dinosaurs have been found in them.
Examples are on display in the Strain hall of Paleontology.
The Cenozoic Era, as the interval from 65 MY ago to present is called, was a time of major change in this region. Early in the Cenozoic, mountain-building forces again were in action and bodies of molten magma moved into the crust. Emplacement of the plutons took place 45 to 50 MY ago. Shortly before plutonism occurred, immense compressive force developed to the southwest and as a result, great masses of Cretaceous limestone were thrust from the southwest to the northeast as a result. These masses constitute the rocks of the Sierra de Juarez.