Is there any London Clay in the East Dulwich & Nunhead valley?

[edhistory]

For nearly a year I have been studying the geology of the East Dulwich & Nunhead valley and I have encountered an unexpected problem.

I have not found any hard evidence that there is any London Clay in our valley.

Over the years there have been posts on the East Dulwich Forum about insurance companies asserting that houses in our valley are built upon London Clay and either refusing buildings insurance or asking for higher premiums.

Has any insurance company provided evidence to support this assertion?

John K

EDIT 2011-06-19: Temporary change of thread title

EDIT 2011-06-24: Title reversion

[Huguenot]

I'm not sure that's quite how it works.

I think that the insurance companies base premiums on the prevalence of particular claims currently being made in an area. If premiums are high, that's because claims are high rather than because of geological studies.

I think the 'London Clay' rationale was created after the fact, and it's more a popular meme than geologically accurate.

The London Basin Forum seems to be the group most likely to generate an accurate map, and it makes clear that London Clay is not always the surface geology, which can be a mix of river terraces and various sandy gravels.

[steveo]

There's plenty in my garden John

[edhistory]

Thanks, Hugenot,

I think you have the essential truth of it. Insurance companies make unsupported assertions and "London Clay" is a convenient, but inaccurate, handle. However, I'm still interested in reports of any assertions insurance companies have made about local geology.

Evidence of any geologically induced subsidence in our valley is elusive.

I have read nearly all of the material of the London Basin Forum (and the Thames Gateway Project) and followed up many of the bibliographic references in the hope that they might contain material about our valley.

There's not much there:

A few inconsequential and contradictory references to the Dulwich Fault.

An analysis of the Harders Road borehole which, although immediately outside our valley, provides tentative evidence for the River Peck alluvial fan.

This is not surprising as apart from the River Peck draining into the Thames Basin our valley is a separate geological feature. I recently placed a map in the Lounge which illustrates this.

The river terraces are a north of the Thames feature. I have carefully examined our valley for any unexpected remnants of river terraces but have found none.

John K

[zephyr]

I remember there being quite a discussion on here a couple of years ago when Thames water started digging that huge pipe from Honor Oak Reservoir. There were quite a lot of links about the make up of the ground in the area as people were worried that their homes were going to sink. Cannot seem to find it now

[zephyr]

Actually just found these - unfortunately a few of the links are now broken

http://www.eastdulwichforum.co.uk/forum/read.php?5,197545,page=1

http://www.eastdulwichforum.co.uk/forum/read.php?5,110179,110179#msg-110179

[Carrie]

I thought that historically there had been brickfields in the area (hence the prevalence of restrictive covenants forbidding the burning of bricks on local property titles) - surely there would need to be clay present for the brickfields to be successful

[benmorg]

When I used to live in Nunhead (Ivydale Road) I did a bit of investigation to find out what the subsoil was, for buildings insurance purposes. Someone at the British Geological Survey told me it's clay. Not sure if it's "London clay" or some other kind of clay. There may be other layers of sediment above or below the clay. If you want more detail for a specific site you can buy a geological map from the BGS (http://shop.bgs.ac.uk/Georeports/info.cfm?STD_REP=S002). BGS might be able to give the general picture for the area, if you can find the right person to talk to.

Dulwich & Norwood have very high buildings insurance premiums (about the highest in the UK) due to the high risk of subsidence. I'm no expert, but I suspect shrinking & expansion of the subsoil clay is the main cause.

[Huguenot]

this map givess a bit of a clue to the inconsistency of surface geology:

http://www.bgs.ac.uk/londonBasinForum/images/3dmodeloflondon.jpg

[cubista]

Try:

BGS geology viewer

Open viewer zoom in and at 1:50 000 a key appears. You may need to adjust the transparancy of the overlay to accurately match the key.

This indicates surface geology to be London Clay over the vast majority (there are no overlying / superficial deposits indicated). To the north towards Bellenden Rd area the geology changes to Lambeth Group clay/silt/sand aka Woolwich & Reading beds, believe this is also known as brick earth; the combination of clay and sand being fundamental to brick manufacture (I think)

[copleston_charlie]

Is there no borehole data from that Thames Water extension to Honor Oak?

[katie1997]

"inconsistency"?

Changeable, yes.

Variable, yes.

Inconsistent, nah. The correct terminology is lateral heterogeneity.

JohnK - someone has suggested you contact BGS which is what I would recommend too, all their maps are online now. I have also PMd you about someone who knows quite a bit about London's geology who may be able to help. Good luck with your project.

PS Here's a favourite of mine from the great, VC Illing, 1919.

"Geology is not an exact science. The personal factor looms so large in many of its problems that he must needs be hardy who dares to be positive, and inexperienced who considers his verdict the final solution."

[The Minkey]

Lot of clay in my garden too. The soil gives way to a solid band of clay about 2ft down.

[dennis]

try telling the guys who shifted skip loads of it whilst building my extension that it wasn't clay!

[edhistory]

I've not forgetten this thread.

It will take me a little time to prepare a response to your contributions.

Sorry.

John K

[PeckhamRose]

But I do prefer the new title that you gave it John!

[Frankito]

Oh yes, so much better.. I will sleep easier tonight lol >:D

[EDmummy]

I seem to remember reading about brick works in ED. Am I mistaken?

[KidKruger]

I dug footings for an extension on Crystal Palace Road and 18 inches-2ft down it was clay, I dug a foot of clay out along the whole 'U' shaped trench. Do NOT tell me that I didn't need to dig down 3ft for a single story extension !

Carrying it out to skip in buckets was murder, clay is a swine to dig and extremely heavy.

So we defo have clay in (at least parts of) East Dulwich.

[edhistory]

EDmummy:

This is the post from last year that you remember.

It was written by that idiot in the bookshop. I'll speak to him, tell him I've read more about the subject than he had, and put him straight.

Every time he wrote "London Clay" he should have written "one or more of the layers of mixed clay, sand and gravel that fall under the umbrella London Clay Formation classification".

That will teach him.

John K