These rocks contain a Golden Spike! Dob's Linn

Geological time is measured by changes in the rocks - this might be radioactive decay for absolute dating, but what is there aren't sufficient radioactive elements, or the rock is made of grains derived from older rocks, as is the case for most sediments?
Relative dating uses changes in the fossil record as the creatures which are preserved evolve through time. A good index fossil is one which is widespread and abundant (easy to find across a wide geographical spread) and evolved quickly (good time resolution). Graptolites are a good example. They were planktonic ocean dwellers throughout the Ordovician and Silurian - in fact, the Ordovician time period was discovered through careful mapping of graptolite zones across the Southern Uplands of Scotland by Charles Lapworth in teh 1890's.
Once you have eatablished a good fossil succession for relative dating and correlation across the world you have to define exactly where the different epochs begin and end. The World standard for the Ordovician-Silurian boundary, the 'Golden Spike" is here at Dob's Linn in the middle of Lapworth's research area.

[@mdc123-v2v]

Good to see you in the Borders! I know we don't have the most exciting geology in the country, but what we do have is pretty special imo!

[@mikelong9638]

I've been watching these from the USA for a couple of years. You have great geology over there. It's just that it rains all the time😂

[@OneMinuteGeology]

Yes, there are loads of classic sites. The problem is we are normally on our way somewhere else.

[@OneMinuteGeology]

@@mikelong9638 true, not quite always. But at least we don’t have rattlers.

[@mikelong9638]

@@OneMinuteGeology You had to mention that!