All slate or Ardesia are created from argillaceous (muddy) sediments such as clay, mudstones and deep sea ooze. Over time the very fine grained clays and mineral particles in these mudstones are buried by younger sediments. As overburden pressures increase water is gradually squeezed out so that flat platey clay minerals and micas rotate and are realigned perpendicular to the direction of pressure so forming a rock known as shale. This dynamic metamorphism over time and with increasing pressure and temperature drives out the remaining free water. Stronger bonding between tighter packed and increasingly aligned particles in addition to the formation of some new minerals leads to the formation of a rock we can now call slate.