You are looking at the largest and heaviest fossil in the collection of the Natural History Museum. You do not need to enter the museum in order to visit GZ. This is part of a primitive tree, discovered in Craigleith Quarry, near Edinburgh in 1854. The rocks from which it was excavated date from the Lower Carboniferous Period, over 330 million years ago. This was the time when reptiles first appeared and much of the land was covered in a swampy forest. The land upon which this tree grew was, at that time, somewhere near the equator.
The petrification process occurs underground, when wood becomes buried under sediment and is initially preserved due to a lack of oxygen preventing decomposition.
What is Petrification?
In geology, petrifaction or petrification is the process of replacement of hard parts of an organism by filling its pores with mineral, making it a fossil. Petrified wood is a common result of this process, but all organisms, from bacteria to vertebrates, can be petrified. Petrification takes place through a combination of two similar processes: permineralisation and replacement. These processes create replicas of the original specimen that are similar down to the microscopic level.
What are fossils?
A fossil is any evidence of life that has been preserved in rock. So fossils include not just organisms themselves, but also the burrows, marks and footprints they left behind. Fossilisation is the name for a number of processes that produce fossils. One of those processes is mineral replacement.
Petrification through replacement
Replacement, the second process involved in petrification, occurs when water containing dissolved minerals dissolves the original solid material of an organism, which is then replaced by minerals. This can take place extremely slowly, replicating the microscopic structure of the organism. The slower the rate of the process, the better defined the microscopic structure will be. The minerals commonly involved in replacement are calcite, silica, pyrite, and haematite. It is rare to find organisms preserved by replacement alone (as opposed to in combination with permineralisation), but these fossils present significant importance to paleontologists because these fossils tend to be very detailed.
Your Tasks
To log this cache, you will need to complete the tasks below. You can use information from the cache page and your observations at GZ to do this. Please message us with the info (rather than post it on your log). You can log a find once we have been sent a message; there is no need to wait for a reply. We may delete your log if your answer is way off the mark.
1. Describe the appearance of the objects at GZ e.g. colour, height, diameter, number...
2. Describe the petrification of wood in your own words.
3. Was this tree petrified through the process of a) permineralisation and replacement or b) just by replacement ? Please explain your choice by referring to your observations of the fossil.
4. As an optional extra post a picture of yourself and/or your GPSr at GZ.