The current building known as Kingsbury Mill was built in the 16th century but a mill at this location is mentioned in the Domesday Book.

It was modernised in the 19th century and was a working mill until 1960, powered by the waters of the Ver.

Geological puzzle
On the grass at the front of the Mill, you will see what at a quick first glance looks like a lump of concrete, but it is in fact a large piece of Hertfordshire Puddingstone.

Hertfordshire Puddingstone au naturelle! Walk Picture Gallery 1

This rock is well-known throughout the world, and can ONLY be found in Hertfordshire with outcrops in some adjacent counties. But, although there are many theories, nobody really knows why. It's a great geological mystery, so go and take a closer look!

It just looks like a lot of ordinary stones stuck together, loosely resembling a fruit pudding - hence its name! But the geological process that led to this is evidence that St Albans was once a tropical area!

Hertfordshire Puddingstone is known as a conglomerate, which means it's composed of pebbles of an earlier rock type incorporated into finer sediment, in this case mainly quartz sand. The pebbles in it are flints derived from the underlying chalk which have been rounded by continually being carried along in a river.

The unusual thing about Hertfordshire Puddingstone is that the pebbles have been cemented together with silica, which is a very hard material. This silica has been drawn out of the underlying rock by water percolating from the surface of the earth downwards.

When the traveling silica found this accumulation of pebbles and sand on its way down, it was re-deposited as a fine material which invaded all the little pores between the pebbles and the sand particles and stuck them all together.

But why did this process only happen around here?
It's actually a bit of a mystery but there are various theories. The silica must have originally come from soils near the land surface, where the rainwater percolating through the soil would slowly dissolve the silica out and carry it downwards.

Hertfordshire Puddingstone polished! Walk Picture Gallery 1

But that is a process which is typical of tropical regions, so it must date back at least 50 or 60 million years ago when the climate in this part of the world was something near tropical - very different from anything that we've had since.

Some people say that the silica was much deeper within the earth's crust so when there were other layers of sediment deposited above it, perhaps 100 metres thick, there was a lot of pressure there to assist these chemical changes. But it's a puzzle!

If you look at a cross section of the rock, you will see a range of very attractive colours - bright yellows, browns and bright reds. This is because along with the silica, iron was also transported through the soil in solution and also deposited in the rock, creating lovely colours.

It is a very hard rock and was used by the Romans to grind corn.