The starting point for understanding Norwich geology is to see it from the air. Not the most obvious place to look first, but it helps you to realise that much of the city centre nestles in a valley that is quite a lot lower than the ground all around it.
You see the winding River Wensum, that has fallen a vertical height of more than 30 metres since it left Fakenham some 40 km back upstream. Although the river still has about the same distance again to flow before it finally reaches the sea, it will hardly manage another metre fall all the way to Great Yarmouth. So the lowest parts of Norwich aren't much above sea level. The daily tide gets up here, in fact the city lies at the head of the Yare estuary and the geological shoreline is actually at Acle.
Another thing you might spot from the air is the odd bare patch of sub-soil exposed on sites being made ready for construction - and there have been quite a few of those in recent years. The colour of the exposed soil can vary a lot in different parts of the city, and that's all to do with which layer is being dug up.
When the Chapelfield shopping centre was excavated, there was a large blob of orange soil visible from above (see aerial view left). But a few years previously when Castle Mall was excavated, it was mostly white. Why the difference? And on the lower land between the railway station and the football ground, the Riverside development was responsible for a big patch of dark brown soil - almost black. What was going on?
Of course, these different colours result from the natural soil types occurring in different places and at different levels in the ground.
Unfortunately it's not a simple matter of the soils being in regular horizontal layers sliced through later by the Wensum Valley. Thanks to an ice age or two and past water flow, the soil boundaries wander up and down in a less than predictable fashion. This is especially true of the upper surface of the chalk, which is the oldest geological stratum we're likely to encounter and under-lies the later-deposited soil sequence in all parts of the city (see cross section on left, and photo with Cathedral).
In the Bridewell Museum was a wonderful nineteenth century painting of the Tombland Fair (held where the park above Castle Mall now is) looking down Rose Lane. In the background you can see the Thorpe hillside opposite Pull's Ferry, and unlike today, it's all white. That's because it was effectively one huge chalk quarry with a worked face rising 20-30 metres above sea level.
Only a modest cannon shot away, in the area between Pull's Ferry and the Cathedral, the top of the chalk plunges away to 10 metres or more below sea level, being covered by a thick bed of brown gravel laid down within the last few tens of thousands of years by a faster-flowing version of the present river as it snaked across the flood plain.
On the inside of the river bends, a layer of black squelchy peat sits above the saturated gravel forming grazing marshes next to the river, hence the dark colour of the soil exposed on the Riverside site a little further downstream when it was finally built on in the 1930s.
Up on the higher ground, things are drier and also tend to get a bit more orange thanks to the sands that cover the chalk - these are known as the crag, deposited in an ancient sea and subsequently uplifted to well above present sea level. This was the bright orange soil being dug out when the underground car parks of The Forum and Chapelfield sites were being excavated. At the deepest point, these holes were just about getting down to the underlying chalk.
In the outskirts of the city centre and beyond is a highly variable covering of glacial sand and gravel, originally thrown down by melting ice water rushing around towards the end of the last Ice Age. This now tends to form the highest ground such as Mousehold, Thorpe and also the ridge now followed by Ber Street. Sometimes this stony layer sits above the crag and sometimes directly on the chalk without the crag being found.
For completeness, we must also mention one or two other important soil types including brickearth, a brown sandy clay occurring in the Catton/Sprowston area (you can guess what that was used for), and a firm light grey chalky clay called till or boulder clay (there's a large patch of that beneath the Hewett School playing field). Both of these are thought to have been laid down by huge ice sheets. There are also special hill-wash deposits called 'head' in some places on the steeper slopes.
For many people the most fascinating soil type in Norwich is the most geologically recent - the layer disturbed by Man's busy activities over the past couple of thousand years - we geologists call it fill. Across most of the city centre is a blanket of dark-coloured soil typically extending 1-3 metres below your feet, containing all the drains and buried cables we rely on, old foundations and rubble from countless demolished buildings, waste pits from ancient industrial processes, river dredgings, miscellaneous pottery and bones and a fair amount of medieval dung. Archaeologists just love it. There is more about this layer in a separate article.
© Matthew Williams
PICTURE 1: Aerial view of city centre 2005 © Mike Page
PICTURE 2: Diagrammatic cross section through Norwich city centre © Matthew Williams
PICTURE 3: Soil section exposed at Palace Street 2006 © Matthew Williams
PICTURE 4: Extract from 1871 painting by F B Barwell (Norfolk Museums Service)
PICTURE 5: Princes Street 2009 © Matthew Williams