 When the North American Plate on its slow journey westwards encountered the Pacific Plate approximately 250 million years ago during the Paleozoic , the latter began to subduct under the North American continent . Intense pressure underground caused some of the Pacific Plate to melt , and the resulting upwelling magma pushed up and hardened into the granite batholith that makes up much of the Sierra Nevada . Extensive layers of marine sedimentary rock that originally made up the ancient Pacific seabed were also pushed up by the rising granite , and the ancestral Merced River formed on this layer of rock . Over millions of years , the Merced cut a deep canyon through the softer sedimentary rock , eventually hitting the hard granite beneath . The encounter with this resilient rock layer caused the Merced River to mostly stop its downcutting , although tributary streams continued to widen the ancient canyon . 
