The Vishnu Group had its beginnings about 2 billion years ago in Precambrian time when thousands of feet of ash, mud, sand, and silt were laid down in a shallow backarc basin . . .This is the metamorphic rock now exposed at the bottom of the canyon in the Inner Gorge. Geologists call this dark-colored, garnet-studded layer the Vishnu Schist. Combined with the other schists of this period, the Brahma and the Rama, this makes up the Vishnu Group (see 1a in figure 1). No identifiable fossils have been found in these strata, but lenses of marble now seen in these units were likely derived from colonies of primitive algae.

The Vishnu Group was intruded by blobs of magma rising from a subduction zone offshore as recently as 1.66 billion years ago. These plutons slowly cooled to form the Zoroaster Granite (seen as light-colored bands in the darker Vishnu Schist; see 1b in Figure 1). Some of this rock eventually was metamorphosed into gneiss. The intrusion of the granite occurred in three phases: two during the initial Vishnu metamorphism period, and a third around 1.5 billion years ago. This third phase was accompanied by large-scale geologic faulting, particularly along north-south faults that caused some rifting, and a possible partial breakup of the continent..

Studies of the sequence of rocks show that the Vishnu Group underwent at least two periods of orogeny (mountain-building). These orogenies created the 5 to 6 mile (8 to 10 km) high Mazatzal Mountains (Yavapai-Mazatzal orogeny). This was a very high mountain range, possibly as high as or higher than the modern Himalaya. Then, for over 500 million years, erosion stripped much of the exposed sediments and the mountains away. This reduced this very high range to small hills a few tens to hundreds of feet (tens of meters) high, leaving a major angular unconformity. The once deeply buried mountain roots were all that remained of the Mazatzal Mountains as the sea reinvaded."

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Wikipedia