Monticello Dam is buttressed against the thickest, strongest sandstone beds of the Great Valley Group. (more below)
Geology Guide photo
This part of the Great Valley Group represents the time, at the beginning of the Late Cretaceous, when deposition of sediment in the forearc was at its maximum. Rapid deposition weighs down on the underlying crust and depresses it, as we see today in the northern Gulf of Mexico and other places. It may be that the movement on the subduction-zone fault reversed direction at this time, allowing the Franciscan Complex rocks to the west to approach the surface again after their deep subduction. If so, then a new subduction zone might have established itself farther west. More recent tectonic movements have tilted the beds nearly vertical. The Putah Creek valley is deepest and narrowest at this point, making it a natural site for a dam.
Day 1, Coast Range: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
Day 2, Sierra Nevada: 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18
Day 3, Sierra Nevada: 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28