These Early Miocene volcanoes, along with the Northland Allochthon, eroded and deposited most of the material that makes up the Waitemata sandstones and mudstones in the Waitemata Basin. The Waitemata sandstones and mudstones form the cliffs around the Waitematā Harbour and East Coast Bays, and land further north up to Cape Rodney, with outcrops further south down to Mercer and Miranda. Lahars produced the coarser Parnell Grit, found in many headlands around the East Coast Bays. Greywacke pebble beaches can be seen forming a layer in Waitemata sandstone cliffs on Motutapu Island. Shortly after (18 Ma), a volcanic arc developed further east to create the Coromandel Ranges and undersea Colville Ridge. The initial activity was andesitic, but later became rhyolytic (12 Ma). In the Kauaeranga Valley, impressive volcanic plugs remain, as does a lava lake that now forms the top of Table Mountain. Active geothermal systems, similar to those that now exist near Rotorua, were present around 6 Ma, and produced the gold and silver deposits that were later mined in the Coromandel gold rush. Later (5-2 Ma), volcanic activity moved further south to form the Kaimai Ranges. The Hauraki Plains and Hauraki Gulf are believed to represent a rift valley. The Coromandel Ranges used to be much closer to Auckland, and the rift valley is assumed to have developed about 2 Ma ago, due to the clockwise rotation of the eastern North Island, that stretched the land between Auckland and East Cape. The Waikato River used to flow from Karapiro, through the Hinuera Gap, and Hauraki Plains, to come out on the east coast in the Hauraki Gulf. The sediment from Oruanui eruption of Lake Taupo caused the river to change direction, and come out to the west. Rivers around Auckland, that now flow to the east, such as the rivers around Clevedon, used to flow to the west, and deposited rocks from the Coromandel Ranges in the Manukau Harbour. The Auckland Volcanic Province comprises the Auckland, South Auckland, Ngatutura and Okete volcanic fields. Intra-plate basaltic volcanism in the Auckland region started in the south, at Okete, near Raglan in late Pliocene times (2.7-1.8 Ma), and has moved north through the Ngatutura, South Auckland and Auckland fields since then.