This invasion is possible due to the deglaciation which then caused a eustatic rise in sea level. It is possible to recognise up to five major cycles of transgression by studing the salt deposits whichnare precipitated out in a set sequence. This occured as the coninents continued to fuse together after the Hercynian Orogeny. There was also some continued vulcanism in SW England, the Midland Valley and the North Sea. In general there was an increase in land area as many areas were fused together.
Most Triassic deposits are typically terrestrial.
Triassic salt deposits can also be found, especially in Cheshire and they were believed to have been formed in a smaller sea or large lake on the opposite side of the Pennine Ridge. One of the most famous episodes in the Permo-Triassic is the mass extinction that occured. It involved the extinction of most of the major groups of organisms that had dominated the Palaeozoic seas in a time of less than 15 million years. Many of the organisms that wre not made extinct were rapidly reduced in numbers. Despite these extictions there were also some organisms that were unaffected and this causes some curiosity as some were morphologically similar to some of those that had died out, E.g. ammonoids died out where as nautiloids survived.