So there you are, doing a nice little resistivity survey on the chalk hills, and you get this :
Wow, you think. Look at all those lovely features. There must be a lot of big pits there, and so you dig them up... and wish you hadn't. Even as you excavate them, you find what looks like an obvious cut and fill in the chalk, but it is only a ruse, for this is a particular form of geology called clay-with-flints. At one time, there used to be a layer of tertiary geology on top of all that chalk, mostly sandy or clay, for example, The Reading Beds. Most of this has been washed away over time, but some layers are still intact. In other places, these tertiary layers, being acidic, have eaten down into the chalk below, creating pit like features filled with clay and flints, hence the name.
This is of course intensely annoying when you are trying to find archaeology, because these features show up particularly strongly compared to any weaker looking archaeology next to it. This cloud does have a silver lining though. You wont find any features with a magnetometer on chalk, as it can't tell the difference between the soil and the bedrock, but if there is clay-with-flints in the area, the clay in the soil will help show up those cut features. You just have to spot them amongst all of the geology.
I have learnt a fair amount about the local geology from doing geophysics, and I will be sharing some of the quirks in this blog.