During the night, the eruptive column which had risen into the stratosphere collapsed down onto Vesuvius and its flanks. The first pyroclastic surge, formed by a mixture of ash and hot gases, billowed through Herculaneum at 160 km/h. It reached the beach and the boat houses, where those waiting for rescue were killed instantly by the intense heat, despite being sheltered from the direct impact. A succession of six flows and surges buried the city's buildings from the bottom up, causing them little damage.