Ipswich Icons: A look back at of the foundations of Ipswich
- Credit: Archant
As well as looking at the past, John Norman today looks down. A long way down. A long way back
This week we go to the very foundations of the town, and establish not when it was built but what it was built upon.
A lesson in geology would ordinarily require a multicoloured map explaining the predominant subsoils beneath our feet; I’ll try and explain what’s down there using a few simple words.
Firstly, how do we know what is below the surface, 10 or 20 metres below ground level? The answer is that we’ve been investigating the geology of the region since the eighteenth century, by digging trial holes, drilling boreholes and recording the results for all to see.
I have been given sight of the bore hole records from the ground survey carried out before the “new” lock was constructed as an entrance to the Wet Dock. The lock was opened in July, 1881, by Joseph Chamberlain (accompanied by mayor Frederick Fish).
In Suffolk the underlying “rock” is chalk, the deposited remains of millions of sea creatures laid down about 100million years ago in the Cretaceous Period (when dinosaurs roamed the planet). As these creatures died they formed calcareous ooze which, when the sea disappeared, became chalk.
Locally, this chalk is very close to the surface at Needham Pits, Great Blakenham quarry and Offton, close to the Limeburners public house. In Ipswich, however, the chalk has been overlain with glacial sand and gravel. It is likely that Ipswich was the southern limit of the sheet ice that covered the country in the last Ice Age.
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As the ice melted (and the weight was taken off) the land rose to be above sea level, the chalk dried and solidified, but then the land sank and the sea once again covered the chalk – depositing a layer of silicilastic mud, silt and clay. In places, the clay was further covered with red crag (Suffolk Crag) – a mix of sand and gravel.
The inland boundary of these deposits varies but, broadly, the clay subsoils occupy the higher ground north west of Ipswich (High Suffolk), with the sandy soils along the coastal margins.
There are places in Ipswich where the geology of the ground below our feet can be seen, exposed by natural erosion and by excavation. Where the overburden of crag meets the London Clay and fresh water springs from the ground.
A good example is Holywells Park, where there is an extensive sand pit (the Dell) just beyond the park-keepers’ yard and springs from where good, clean, fresh water flows to feed the ponds. John Cobbold was aware of the cleanliness of this water when he purchased Holywells House and grounds, using the water to supply his brewery.
Back to the boreholes records. A survey of the ground below the dock to investigate the sub strata prior to the building of the new lock pit in 1878 -1881 reveals the depth of chalk in the valley bottom. Holes were drilled (by percussion boring) in the Wet Dock, into the embankment that separated the dock from the Orwell and into the river itself immediately outside the proposed lock gates.
Eighteen holes were drilled and the results show that the depth at which the chalk is encountered varies widely: from a few centimetres to over 25 metres just a short distance into the river.
These results are borne out by a similar survey carried out prior to the building of the Orwell Bridge (1979-1982) where the depth of the chalk, although generally deeper (as would be expected) still varies from a couple of metres down to 40 metres.
The piles supporting the bridge disappear an additional 10 metres into the chalk (where the chalk is less fractured, and less affected by water). The British Geology Survey has the record of hundreds of boreholes dug into the ground below Ipswich, notably (in this location) those carried out prior to Project Orwell, the flood barrier and the construction of the West Bank Terminal.
Further research has just been completed to establish ground conditions under the proposed Upper Orwell Crossing, a new bridge that will cross the river between Wherstead Road and Holywells Road. Although these results are not yet in the public domain, I understand there is, as to be expected, remarkable similarity with results from similar boreholes dug 130 years ago.
I am indebted to Una Denny, who carried out some of the research for this article as the basis for her dissertation back in the late 1960s.