The lower part of the river, in sharp contrast, is a meandering stream flowing over Cenozoic alluvial deposits (66 MYA-present), which together comprise the flat floor of the Central Valley. The tremendous volume of sediments that underlie the lower San Joaquin River ranges from deep, with distance to bedrock generally increasing in a northerly direction. Prior to the uplift of the California Coast Ranges, more than of sediments were deposited at the foot of the Sierras by tidal activity, and the ancestral San Joaquin and its tributaries flowed west over this alluvial plain to the sea, dumping their own sediments onto the marine deposits. Compressional forces along the boundary of the North American and Pacific Plates between 2-4 MYA resulted in the uplift of the Coast Ranges, creating an enclosed basin today known as the Central Valley and resulted in the San Joaquin's present path to the sea.