During the early and middle Ordovician period (485 to 460 Mya), volcanic activity increased. One large volcanic system, which was centred around what is now Snowdon, emitted an estimated of debris. Another volcano formed Rhobell Fawr near Dolgellau. During this period, great accumulations of sand, gravel and mud were deposited further south in Wales, and these gradually consolidated. Some of the volcanic ash fell in the sea and formed great banks, where unstable masses sometimes slid into deeper water, creating submarine avalanches. This caused great turbidity in the sea, after which the particles began to settle out according to particle size. The strata thus formed are called turbidites, and these are common in central Wales, being particularly obvious in the sea cliffs around Aberystwyth.