Fossils on Friday – Maps and Cross-sections

Welcome back to Fossils on Friday, as we look once more at maps! Last time might have seemed a bit complicated, but hopefully this post will clear a few things up!

First, let’s remind us of the maps from last time:

Geological map 1: Here it shows you different rock types , as well as the elevation. The dotted lines show elevation – the numbers tell us how high it is. So the hill tops are here:

The hill tops – with an elevation of 800 feet on the left, and 700 on the right.

The valleys are the dotted lines with the lowest numbers – often coming together, making a point. So the valleys are here:

The valleys, both with a low of 400 feet elevation.

So, the lowest points are where the contour lines are shown to be the lowest number. With small valleys running around the three hills.

Can you start to see the 3-D structure now? Now let’s re-cap the geology. The solid lines mark the different geology – grit, limestone and shale. Like this:

Now, look at the limestone. On the left, there is some exposed at the elevation of 800 feet, poking out at the top of the hill. However, by the time you get to the centre and right of the map – the limestone exposure is down to 400 -700 feet.

This must mean that the limestone bed itself is sloping downwards. It sits on top of the shale – you can see that on the left hill. But the grit sits on top of the limestone – you can see that on the right hill. So all of the deposits are sloping downwards. The only way we can see any of them, is because of the valley – which cuts down through the layers of rock, making a cross-section for us!

Now, I want you to imagine that the map above is a huge cake, made up of 3 layers – chocolate, strawberry and vanilla. Now, we’re going to take a great big knife, and cut through the cake, along the A-B line.

A great big slice across the map cake – right along the A-B line!

So – what would this clean slice look like? Well, something like this:

This is a clean cross-section through the A-B line. You can see at the top of the hill on the left side, that there is the limestone exposure, leading down towards the valley, where the bottom layer of shale is exposed again. The limestone is exposed again as we move back up through the elevation, before moving into the grit, lying above the limestone.

Can you see how all the layers are sloped? So where the valley has been eroded away, it reveals the lower rock sequences, down to the bottom layer.

So, hopefully that has cleared some stuff up for you! Next week well be looking at how to draw a different type of map – looking at rock exposures near you.