Sometimes, scientists already know the age of the fossil because fossils of the same species have been found elsewhere and it has been possible to establish accurately from those when the dinosaur lived.
Geologists call this the principle of lateral continuity.
There are some radioactive elements in rock that decay by giving off energy and turning into different, more stable elements.
In the process of disintegration, the atom gives off radiation (energy emitted in the form of waves). Each element decays at its own rate, unaffected by external physical conditions.
By measuring the amount of original and transformed atoms in an object, scientists can determine the age of that object.
Scientists know exactly how long it will take for half the quantity of the element to change, and this state is known as its half-life.
After another half-life has passed, the element will have decayed to a quarter of its original amount.
Cosmic rays: Invisible, high-energy particles that constantly bombard Earth from all directions in space.
Dendrochronology: Also known as tree-ring dating, the science concerned with determining the age of trees by examining their growth rings.
This is because new sediments are always laid down on top of sediments that have already been deposited.
So, when looking at the history of a cliff face, it is important to read the story it tells from the bottom layer up.
The Age of Dinosaurs was so many millions of years ago that it is very difficult to date exactly.
Scientists use two kinds of dating techniques to work out the age of rocks and fossils. This considers the positions of the different rocks in sequence (in relation to each other) and the different types of fossil that are found in them.
The sediment of this area was laid down after ammonite A appeared 199 million years ago, and before ammonite B became extinct 195 million years ago.