At around 1:00 pm, Mount Vesuvius began spewing volcanic material thousands of metres into the sky. When it reached the tropopause, the top of the column flattened, prompting Pliny to describe it to Tacitus as a stone pine tree. The prevailing winds at the time blew toward the southeast, causing the volcanic material to fall primarily on the city of Pompeii and the surrounding area. Since Herculaneum lay to the west of Vesuvius, it was only mildly affected by the first phase of the eruption. While roofs in Pompeii collapsed under the weight of falling debris, only a few centimetres of ash fell on Herculaneum, causing little damage but nonetheless prompting most inhabitants to flee.