We all know chalk in one way or other. For me it was sticks of it exploding on the wall behind me and a teacher screaming “Pay attention Harries!!!”. For others maybe just something they played with when young or what we use to write on pavements and walls. And we know then that we can write with it as it’s soft. And we know it dissolves easily as it makes the sea chalky white at Peacehaven and Seven Sisters. But. Some chalks you can build with. Chalks and Limestones are sedimentary rocks, made from calcium carbonate, itself made from microscopic sea creatures, algae called Coccolithophore that after dying settles on the ocean floor and after tens of millions of years were compacted into rock. And different layers have different strengths and while most limestones are strong enough for building, and while soft chalk has also been used for building and in dry situations can last, only really 1 layer of the chalks of Britain is strong enough for building with and was used in building London and that’s called Clunch and it’s most famous pace of extraction is at Totternhoe on the Dunstable Downs.  
“The stone, although a chalk, is soft when first extracted but becomes unusually hard after drying out, due to partial cementing with silica. Thus it is easily worked initially but makes for a usable but not high quality building stone.” 
Totternhoe Stone is about the oldest of the Chalks, though compared to the granites of the Embankment are 280 million years old, Totternhoe Stone is pretty recent being only 96 – 94 million years old! (Deep time is great! 😀 ) 
“Totternhoe Stone is a bed 2–4m thick of hard cream or greyish-cream rock within the Lower Chalk (see the Bedfordshire Solid Geology Map). During the deposition of the Chalk there were pauses in sedimentation that encouraged a greater amount of cement to form on the seafloor. These thinner beds are often called ‘hardgrounds’ because of this property. The Totternhoe Stone is a bed that has acquired more cement than the usual Lower Chalk (it can also be slightly gritty). Despite being very prone to damage by frost and acid rain, it has been a popular building stone since Roman times – where there are few other building materials! Its popularity for use in buildings near its outcrop lies in the fact that it is easy to locate, easy to quarry, soft enough to make decorative carvings, and cheap. The main quarries are along the slopes between Totternhoe to Sewell, including underground workings of the late 1700s to late 1800s. Where used inside buildings Totternhoe Stone survives the ravages of time very well. It has often been used inside Church porches or even in the alter back of Westminster Abbey. However, when used within the outside
building structure on older buildings, this rock is often badly weathered.” 
You can see in the profile Totternhoe is in the middle of the Lower Chalk. As it’s less permeable and on top of Greensand, springs often come out where the clunch is. It has more clay or silica in it than younger chalk and this is what makes it stronger. 
And I think it’s place in the profile means that the old quarry faces I have photographed can not show Clunch, though some of the boulders may well be, but simply much younger chalks.
Tbh this post is a reach in terms of ‘building London as there is not that much evidence of it being used much in London. It has been found though under the Edgware Road “A block of Totternhoe stone from near Dunstable, a boulder of granite, and a sandstone block, possibly from the boulder clay, occurred in the paving.” in what is thought to be a Roman paving, [ nb Edgware Rd is the Roman Watling Street ) which suggests that the Romans were bringing this stone into London,  while others say “Totternhoe Stone and Clunch, from the lower chalk formations of Bedfordshire and Hertfordshire, were restricted to vaulting in London; and chalk for the webs of the vaults of Westminster Abbey is mentioned in the accounts of 1253: and as walling, chalk was used in the crypt of All Hallows, Barking, Church.” 
But then it was also used in the White Tower window jambs and I suspect that it appears in other Roman sites in London.  and chalk is also repeatedly mentioned in mediaeval churches in the City of London, and it’s not clear if that could include Totternhoe Stone. It may just be chalk from Woolwich and Plumstead though which would more easily make it’s way to the City. 
Where Totternhoe Stone was was quarried is know called the Totternhoe Knolls, which are in fact old spoil heaps,and now a nature reserve. There were later mines or adits going into the hillside though these have now all gone, either having been mined out for chalk extraction for lime and cement or they have been covered up by yards of spoils and are not accessible. 
In the last decades the stone has been quarried only from one small pit for small scale building works including repairing Woburn Abbey. The site is now an SSSI called Totternhoe Stone Pit, a Geological Conservation Review site, inaccessible to the public.  
And you can buy a ton on Ebay for £600! 
The whole site is great to visit, from the incredible views from the castle hill, to the great chalk downland flora. Large areas of the site are now owned and managed by The Wildlife Trust for the chalk grassland flora. The Castle, a Norman earthwork is managed by the Natural Trust  
Totternhoe is near Dunstable so near Luton. To get there from London get a Thameslink or St Pancras train to Luton then use the Busway, that follows the old railway line to Dunstable. If you can get one to Beecroft. Totternhoe is then a short walk.
Cycling; there is a cycle way that goes next to the Busway from Luton to Dunstable and apart from an missing section then to leighton Buzzard, and past totternhoe
Driving. M1, A5 and A505 from London. There’s a strangely un-signposted National Trust Carpark off Castle Hill Road by a bustop.
https://www.wildlifebcn.org/nature-reserves/totternhoe – there’s a good map here. http://www.letsgo.org.uk/unwind/Totternhoe_Knolls.aspx https://www.chilternsaonb.org/ccbmaps/705/137/totternhoe-knolls.html
https://www.bedfordshiregeologygroup.org.uk/uploads/1/3/2/1/132121510/blgg_totternhoe_stone.pdf  https://www.bedfordshiregeologygroup.org.uk/geology.html  https://ougs.org/london/event-reports/568/field-trip-kensworth-quarry-dunstable-downs-and-totternhoe-quarry/  https://ougs.org/london/event-reports/568/field-trip-kensworth-quarry-dunstable-downs-and-totternhoe-quarry/ https://www.bedfordshiregeologygroup.org.uk/uploads/1/3/2/1/132121510/rigs_building_stones.pdf  https://www.bedfordshiregeologygroup.org.uk/uploads/1/3/2/1/132121510/blgg_cretaceous_downs.pdf  https://penelope.uchicago.edu/Thayer/E/Gazetteer/Periods/Roman/Topics/Engineering/roads/Britain/_Texts/CODROM/2*.html  https://www.british-history.ac.uk/rchme/london/vol5/xxiv-xlviii  https://www.yumpu.com/en/document/read/11957881/medieval-building-stone-at-the-tower-of-london  https://www.academia.edu/10933444/Saxon_and_medieval_parish_churches_in_the_City_of_London_a_review  http://www.disused-stations.org.uk/s/stanbridgeford/index0.shtml
 http://www.clunch.co.uk/  https://www.ebay.co.uk/itm/165031127541?mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338722076&toolid=10001  https://historicengland.org.uk/listing/the-list/list-entry/1020772  https://www.wildlifebcn.org/nature-reserves/totternhoe
https://www.stonespecialist.com/news/great-british-stone-totternhoe-clunch http://bucksgeology.org.uk/pdf_files/Livelihoods-from-Chalk-Beds-2011.pdf https://astonrowant.wordpress.com/chiltern-hills/totternhoe-stone/