Portland Stone
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.
The more shelly stone is referred to as Roach Stone and is mainly found in the base bed.
Roach Stone
Roach stone is rich in fossils (known as shells to local masons) often consisting of almost fifty percent fossilized remains with an average size of approximately 350 mm. The stone, has a maximum bed height of 2 metres. The stone is renowned for being highly frost resistant and being able to withstand the chemical effects of seawater. Due to its high shell content it cannot be easily carved so is used mainly for decorative panels.
In order to substantiate your visit and be able to claim this Earthcache, a number of questions need to be answered and submitted to the cache developer, via e-mail.
1. Observe and describe the Portland Stone that makes up Nicodemus Knob. Include such things as colour, texture and general appearance. From your description would you say that it was sedimentary rock (formed by deposition and solidification of sediment) or igneous rock (formed by the cooling and solidifying of molten materials)?
2. Observe and describe the Roach Stone on the Northern Edge of Nicodemus Knob. Include such things as colour, texture and appearance. How does it differ from the Portland Stone of Nicodemus Knob? What marine life do you think made the shapes and texture of the roach stone?
3. The Roach Stone has a large crack in it. How long would you say the crack is and does it run horizontally or vertically?
NOTE: You may log your visit prior to approval, but e-mail submissions that do not meet the above criteria will be deleted.
Cache placed with the kind permission of Natural England. Please be careful not to trample the orchids or species-rich limestone grassland around GZ.