In a geologic context, the San Joaquin River can be divided into two major segments. The upper above Friant Dam in the Sierra is characterized as a steep-gradient, rocky mountain stream. Over millions of years, the upper San Joaquin, as well as the upper reaches of most of its tributaries, have eroded enormous amounts of rock and sediment from the mountains. Most of the Sierras are underlain by granitic igneous and metamorphic rock dating back to the Mesozoic Era (250-66 MYA); in addition many of the San Joaquin's tributaries flow across a foothills region of metamorphosed volcanic rock more famously known as the Mother Lode Gold Belt.