The Stanislaus River is believed to have originally formed sometime during the Miocene period, about 23 million years ago, flowing down from an ancient mountain range in the current location of the Sierra Nevada that has since eroded away. Huge lava flows moved down the ancestral canyon, filling it with volcanic rock and sediment. About 9 million years ago during the Pliocene, the most recent period of orogeny (uplift) occurred, tilting the predominantly granitic Sierra Nevada batholith to form a regional slope to the west. As the mountains rose the Stanislaus River established its modern course, carrying away the volcanic material that had filled the ancestral canyon. This orogeny coincided with a second period of volcanic activity, during which lava flows displaced the Stanislaus River course several times where it flows through the foothills, causing it to carve new canyons through underlying sedimentary rock. The lava flows are today known as the Stanislaus Formation, and are most visible as the caprock layer of the distinctive "table mountains" around present day New Melones Lake.