London is underlain with rock, but it’s not the type of rock that we generally think of as rock. Most of London sits on up to 140m of clay, though a few areas in south-east London sit on chalk. And while technically rock, clay needs to be baked to become a building material, brick, and chalk is generally too soft for building with. The London area did also have a few random rocks that can be used for building, ferricretes, gravels bound together by leached iron, and some rare glacial deposits but basically, London had/s no building rocks readily available to build with, as anywhere on granite, limestone or sandstone does.
So, London has always imported stone for building and before Portland became London’s default building stone after the Great Fire of London, Kentish Ragstone was London’s premier building stone for 1500 years or so. All the Roman and Medieval buildings in London have some Kentish Ragstone in them. We know there is Kentish Rag in one of the oldest buildings in London, the 3rd century Roman built London Wall  and we know that Roman boats carried it from quarries in Kent to London In 1963 excavations at Blackfriars discovered a Roman boat that had sunk, that was ” carrying a cargo of building stone by the rivers Medway and Thames from the Maidstone area of Kent, south-east England, to London. ” How and when the Romans discovered the Medway stone I guess we will never know but they used vast amounts of it as did future generations.   
Simon Ellis has done lots of outstanding work on Roman quarries in Kent, and the use by the Romans of that stone in London, estimating that “The walls included more than one million squared and well dressed ragstone blocks needed for the facing, an overall ragstone volume of 35,000 cubic metres, requiring … 1,750 voyages of medium sized merchant ships to transport the stone the 127km from the quarries to London.” [ I had to check that 127Kms figure as it seemed so high and measured from to 120km from Dean Street so it is accurate! Of course the boats would have had to go all the way down the Medway, then through the marshes and then all the way back around and up the Thames. Of course, as the crow flies it’s only 50Km/30 miles! ]   It’s importance was so great that Simon Ellis has called Kentish Rag, “London’s Grey Gold’, as here in this interview with The History Girls blog. 
The ragstone was historically quarried to the south of Maidstone and while evidence on the ground of the Roman quarries is sparse, but there is much evidence of more recent quarrying, much of which probably ended up in London, and I’ll post about this in Part 2. There was also a significant revival of using ragstone in London in the 19thC , to give a Gothic Revival look to churches, and I have seen Kentish Rag used for stone work on the Lea Navigation and Bow Back Rivers.
British History Online makes clear the importance of the stone – “Without doubt [ Kentish Ragstone ] takes priority in any description of masonry in London, for it was employed more than any other stone. The Roman wall was mainly built of it, and throughout the middle ages this stone was almost universally used for the bodies of the walls of churches and other important buildings, such as the Tower of London, the Guildhall and the Halls of City Companies. ” 
And there is another sunken cargo boat at Blackfriars, ‘Blackfriars 4’, found in the 1960s and this time it was “… probably of the fifteenth century, .. with a cargo of Kentish ragstone lying close to Blackfriars wreck 3. … overlying the frame in the bottom of the vessel was a spread of irregular lumps of Kentish ragstone, some of which were quite large (up to 0.5m across). This is presumed to be the cargo. ..The stone was no doubt quarried in the Maidstone region and had been brought down the River Medway and up the Thames to London where it was presumably intended for building purposes. The Bridge House weekly payments for 1381-1405 (Appendix 6) include a record of many ship voyages that brought ragstone from the Medway to London, from which it is possible to place the wrecked vessel in its context. Since the normal type of vessel that carried ragstone to London was called a farcost during the fourteenth and fifteenth centuries (Wright 1988, 151; Salzman 1967, 350), it is possible that Blackfriars ship 4 was such a vessel..”  
But what is Kentish Ragstone? Unfortunately It’s not entirely straightforward! Sometimes it is described as a limey Sandstone and sometimes as a sandy Limestone, and it varies from area to area, quarry to quarry and bed to bed. Bernard Worssam and Tim Tatton-Brown in their authoritative ‘Kentish Rag and other Kent building stones’ call it a “… hard sandy limestone, consisting of rounded detrital grains of quartz and of the green mineral glauconite, cemented by calcite (a crystalline form of calcium carbonate). ” occurring “ .. in the Hythe Beds formation, which is part of the Lower Greensand,  a group of formations of Lower Cretaceous age.” 
John Allen Howe in his Geology of Building Stones, 1910, states “In rocks like the Kentish Rag, there is every gradation from a sandstone with abundant calcareous cement— mainly of organic origin—to a shelly limestone with scattered sand grains.” 
And Graham Lott and Don Cameron state in ‘The Building Stones of South East England; Mineralogy and Provenance’, “The Kentish Ragstone is so variable in its lithological character that it is difficult to select a few samples ‘typical’ of the many varieties that may be encountered in the outcrop. The two best known lithologies are the Ragstone and Hassock (Worrsam 1963). The Ragstone is a hard, bioclastic limestone with sparse detrital quartz grains and glauconite…. Much of the medium- to coarse grained bioclastic debris present is ferroan spar-replaced, and identifiable fragments are restricted to the surviving non-ferroan grains. These include common bivalve debris, echinoid plates, small benthonic foraminifera tests, ostracod valves, sponge spicules, bryozoan and algal fragments. The sparse monocrystalline quartz and green glauconite grains are of very fine to fine sand grade. In other sections examined from the Ragstones, sponge spicule debris is more common, sometimes forming pervasive, polycrystalline siliceous patches.”  
Lott separately, in his “Sourcing stone for the conservation and repair of the buildings and monuments of Britain”, calls Kentish Ragstone a “hybrid” stone – “There are a number of hybrid stones comprising a variable mix of carbonate (bioclasts, ooids etc) and terriginous grains (quartz, feldspar etc), usually with a carbonate cement or micritic matrix, most notably perhaps including the Kentish Ragstone (Lower Cretaceous).” 
And the British Geological Society County Atlas states “Kentish Ragstone is typically a medium-grey limestone, occasionally with associated nodular or bedded chert layers. Variations in the proportions of quartz grains, glauconite and carbonate cement/matrix have contributed to the differing workability and weathering characteristics of the stone… Although the generic term ‘Ragstone’ denotes the relative difficulty in working the material to produce dressed stone, historic examples of more high-quality dressed stone and tracery are evident throughout the county. However, Ragstone is more commonly seen as rubblestone walling, either coursed or uncoursed.” 
As the BGS state, Ragstone has mainly been used for “rubblestone walling” not ‘ashlar’, or block walling and that’s what you will see in numerous Mediaeval and Victorian churches in London as well as in The London Wall though conservation repair company PAYE state – “Although traditionally difficult to ‘dress’ with a regular face, ragstone has been used for blockwork in the construction of walls and buildings of some of London’s finest properties since the 11th Century; including The Tower of London, The Medieval Guildhall and Westminster Abbey. The building of The Abbey in the 1240’s required such large quantities of ragstone with the result that all local supplies were commandeered for that purpose. A royal command at the time decreed, that “no Kentish ragstone shall be carted to London for any other purpose until the Abbey is built”.  PAYE have also worked in the restoration of the Roman, and Kentish Ragstone, Temple of Mithras. 
Rubble building was the accepted form of building, to expensive ashlar alternatives such as Caen or until the 1thC when Portland or Bath stone became standard, and all the mediaeval churches scattered across London and it’s now swallowed villages use ragstone and other stones in rubble walls.
John Watson in his ‘British and Foreign Building Stones’ of 1911, noted “Kentish Rag … deserves special notice. It is a rock which from time immemorial has been used as a building stone in the south of England. In the life of Sir Christopher Wren, written by his son, reference is made to the restoration of St Paul’s Cathedral in London, after the great fire of 1666. Wren confidently asserts that the Cathedral then destroyed by the conflagration was the fourth building which had occupied that site, and that each of them was erected on the old foundations, which were of Kentish Rag … Quite a large number of churches were built in London during the early part of the 19th century of a combination of Kentish Rag and Bath or other Oolite. The Kentish Rag is hard and tough, it is therefore difficult to dress, and consequently expensive to use ; it is principally employed for rubble masonry, while the softer Oolite is used for the quoins, jambs and other facings. ” [20 ]
Renown buildings of Kent Ragstone in London, though most included anything the builders could lay their hands on, include the Jewel Tower  on the Isle of Thorny at Westminster and the Abbey itself, and associated College Garden Walls  and the White Tower of London which while mainly built of Caen Stone included much Ragstone as does much of the rest of the Tower of London walls.  A good central mediaeval example is All Hallows by The Tower – “The walls are of ragstone and other rubble with dressings of limestone;”   Guildhall is great as you can see Roman use of Ragstone in the Amphitheatre in the basement of the Guildhall Museum and 15thC walls in Guildhall itself as well as in the magnificent undercroft! 
A typical 19thC example is St Barnabas in Homerton.  block rather than rubble.
Or in Brentford the sadly derelict mediaeval church of St Lawrence “Tower of Kentish rag with ashlar dressing”  with rubble construction.
Or in north London the mediaeval All Hallows at Bruce Castle, St Ann’s in South Tottenham and St Pauls at Northumberland Park. 
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 https://www.kentonline.co.uk/kent/news/what-did-the-romans-do-for-us-188634 /