Portland Stone was formed at the end of the Jurassic period, around 145 million years ago when what is now Portland, was much closer to the equator than it is today.
A chemical reaction in the warm, shallow seas where Portland Stone was forming caused calcium and bicarbonate ions to combine, forming a 'muddy' calcareous precipitate. Minute particles of sand or organic detritus, such as shell fragments, lying on or in suspension close to the sea floor, acted as nuclei which gradually became coated with this fine-grained calcium carbonate.
Over time more calcium carbonate accumulated around these nuclei in concentric layers, forming small calcareous spheres (less than 1mm diameter). Countless billions of these spherical sediments, called 'ooids' or 'ooliths', ultimately became buried and partially cemented together by more calcium carbonate, resulting in the oolitic limestone we now call Portland Stone.
Fortunately, the degree of cementation in Portland Stone is sufficient to allow it to resist the detrimental effects of the weather, but it is not so well cemented that it can't be readily worked (cut and carved) by masons. This is one of the reasons why Portland Stone is so favoured as a monumental and architectural stone.
For more information on Portland's quarries and its stone, please click here.