Geological Time
The Earth formed approximately 4.6 billion years ago. Geologists have created a time scale which divides this time into intervals which represent distinct phases in the history of our planet. Below you can see a timescale with Eras on the top which are further divided into Periods on the bottom. The numbers represent the age in millions of years. This Earthcache series will take you on a journey through the Ordovician, Devonian, Carboniferous and Quaternary periods.
The time spans we are talking about can be daunting and difficult to visualise so I have represented them with their colour on a 24 hour clock. In other words all of Earth history is scaled down to one day. Below you can see that specific events can be given a time on this clock to help us better picture when they occurred. For example the oldest fossils date from 5:20 and humans only appeared at 23:59:56 which means we have only been on Earth for 4 seconds out of a full day. It is worth noting that the most change has occurred late in Earth history. As I introduce each Earthcache I will give the time range to which it corresponds on the 24 hour clock.
This Earthcache requires a compass (can be on smartphone)
Please stay on the path.
EarthCache #2
The Devonian Period lasted from 415 to 354 million years ago. This is equivalent to 21:49 -22:07 on the 24 hour clock. During this time Scotland was in a semi-arid environment above sea level. It was a volcanically active time.
Part 1: At this location you will find two different kinds of lava known as Andesite and Trachyte as well as volcanic ash. The ash layer is about 1m thick and is a rough, rubbly white rock. Andesite is purplish in colour, has closely spaced cracks, a smooth surface and very fine layering. The word Andesite comes from the Andes Mountain where it is common. Trachyte is pale brown and has a rough surface.
Your task is to determine which rock type (Trachyte, Andesite and Ash) corresponds to each letter in the blurred image above.
Optional:
Create a field sketch. Why don’t you just take a photograph you might ask? Geologists draw field sketches to pick out the important features which they would like to record leaving out unimportant details such as vegetation. This makes it easier later on when you want to remember what it is you were looking at.
A good field sketch includes the following information:
-Orientation (using compass)
-Location
-Scale
-Date and time
-Weather (Affects the colour perceived)
-Simplified drawing with rock types
-Colour
You can send me the answers via the message center or by email on my profile page. Please do so before completing the log and do not include the answers in the log. Once you have drawn the field sketch you can either scan or photograph it and send it along with the answers.
Part 2: At 55°55'19"N 3°11'30"W you will see an ice moulded crag and tail form.
This feature is formed by selective erosion beneath an ice sheet. The tail side points in the down ice direction. When the flowing ice encounters a section of more resistant rock it preferentially erodes the surroundings leaving the surface feature we see today. The feature ramps out as the rocks behind the resistant section are protected by it. It was formed during the Quaternary Period which covers the last 2 million years. This is equivalent to 23:59 – 24:00 on the 24 hour clock. During this time the Scottish rocks were reworked by ice.
Your task is to determine the orientation of ice flow in the past using your compass.
Permission obtained from City of Edinburgh Council.
My thanks to Lothian Borders GeoConservation Group for feedback.