I was looking around Botanic Gardens recently, my first visit for many years when I saw some limestone flagstones, so naturally I had a look.

The photo below will help you locate the particular stone to look at.

1) Describe the large fossil obscured by the earthcache symbol in the photo
2) What fossil do you think this is?
3) Is this a good zone fossil? Give a reason for your answer.
4) Take a photo of yourself, or an identifying item, near GZ. Please be careful not to give the answers to the questions in your photo.

Please submit your answers via message though the Geocaching website or by sending me an email - there is no need to wait for a response before logging your find.

Limestone

Limestone is a sedimentary rock formed from the remains of tiny sea creatures and shells. These remains settle on the ocean floor and build up in layers over millions of years. Over time, pressure turns these layers into solid rock. Limestone can be polished to a smooth, shiny finish and is often used for building cladding due to it's attractive appearance.

Zone Fossils in Limestone:

Zone fossils, also known as index fossils, are used to define and identify geological time periods because they are widespread, abundant, and limited to a short span of geologic time. Not all limestone fossils qualify as zone fossils, as some may not meet these criteria.

Limestone Fossils That Are Zone Fossils

Ammonite

Ammonites were marine creatures with spiral shells that lived in oceans, swimming in open waters and using gas-filled chambers for buoyancy. Especially in the Jurassic and Cretaceous periods, they are excellent index fossils due to their rapid evolution and widespread distribution.

Brachipod

A brachiopod fossil is a marine organism's shell, typically symmetrical and resembling a clam. Some species are used as zone fossils in Paleozoic limestone, such as specific Orthid and Spiriferid brachiopods.

Belemnite

Belemnites are considered good zone fossils because their distinct, widespread species lived for relatively short periods within the Mesozoic Era, making them usefulfor dating rocks. These squid-like creatures first appeared in the Late Triassic period and became extinct at the end of the Cretaceous period, around the same time as the dinosaurs. The most abundant fossils are from the Jurassic and Cretaceous periods.

Limestone Fossils That Aren't Zone Fossils

Gastropod

Gastropods are mollusks that lived in marine environments, using their coiled shells for protection as they crawled along the ocean floor or attached to rocks. Though present in many marine limestones, they generally evolve slowly and have broad temporal distributions, limiting their use as index fossils.

Coral

A coral fossil in limestone looks like a rough, circular pattern with ridges and grooves. Although common in limestone, many coral species have long geologic ranges, making them less useful as zone fossils.