Mr Holmes and the subsiding lady

I well remember the remarkable morning of Thursday April 12 1878, during which a gathering of eminent gentlemen assembled in the swirling mists of Blackheath to witness a singular scientific phenomenon. Having arrived early at the suburban hamlet of Blackheath on the 7.17 from Charing Cross, I ascended to the summit of the plateau, being immediately consumed by the fog and uncertain ground occasioned by centuries of gravel digging. That this place in recent memory had harboured vagabonds and highwaymen could not be doubted, and indeed a vintage yet effective pistol lurked in the folds of my greatcoat as a foil against such happenings. As I reached the crest of a particularly deep cavity, a disembodied voice shouted from my right.

"Mr Holmes, is it you? Mr Holmes?"

I recognised in a moment the commanding tones of Sir George Airy, the country's greatest astronomer, who had recently arrived after a short perambulation from his offices at the Royal Observatory a few furlongs distant. The gesticulating body was momentarily lost in the mists, reappearing sudddenly much nearer to me.

"Good Lord, man," he exclaimed through the high folded collars of his jacket, "how we can hope to find one another in this waste land is a mystery. Thank you for arriving at such short notice - we have found something most peculiar, and the cause at this moment is rather beyond our explanation, learned gentlemen as we are."

Taking out my pipe, which had a tendency to concentrate my thoughts, I replied, "Sir George, everything in nature can be explained if we can set our hands on the relevant information, however extraordinary events may be."

We shook hands warmly, and made all haste through the gorse bushes to a cluster of well-wrapped men standing in a circle around some noteworthy spectacle, as a group might worship a bonfire on an icy January night. But there were neither smoke nor flames, and as the group parted for us with welcome salutations, the object of their attention became immediately apparent: a fathomless cavity in the ground about seven feet in diameter, whose bottom could not be seen in the dim grey air that embraced us.

The above is my fanciful but not impossible dramatic reconstruction of the investigation into the Blackheath subsidences of 1878; the Mr Holmes in question is not the great detective (who did not appear in print until the publication of A Study in Scarlet in Beeton's Christmas Annual 1887), but Mr T.V. Holmes F.G.S. - an expert geologist of the time.

The Blackheath subsidences came to public attention when three occurred in fairly quick succession after the extreme rainfall of April 1878. T.V. Holmes is our guide (from The Engineer Feb/March 1881):

"During the night of Wednesday and the early morning of Thursday, April 11th and 12th, an extraordinary fall of rain visited the metropolis and its outskirts, causing an amount of distress from floods which appears scarcely ever to have been equalled in that part of the kingdom. The inundations were particularly severe in the valleys of the Ravensbourne and the Quaggy - a fact to which Sir Joseph Bazalgette bore witness in a report which he made as the engineer of the Metropolitan Board of Works. On the morning of Thursday a singular phenomenon presented itself on the broad open plateau of Blackheath. At a spot near the ride known as "Rotten Row," the earth was found to have sunk in to a depth of about 20ft., leaving what was described at the time as a "dangerous hole eight or nine yards in circumference." [location A on plan] The Metropolitan Board, who had jurisdiction over the heath, first of all placed a fence round the mouth of the shaft, and subsequently filled it in. The fence was then removed, and there was apparently an end of the matter. But early in November last another hole [location B] of a similar character opened in the Heath at a spot considerably to the westward of the first subsidence, and on the 19th of that month a third aperture [location c] presented itself, not far from the first. Parties residing in the neighbourhood began to think that these earth-falls were looking serious, as it was impossible to say where they might ultimately show themselves.

The aperture A is that which first appeared. It is situated about 250 yards from the nearest point of the park wall, close alongside of which runs the Charlton-road. Its position upon the Heath is very central, being about 370 yards E.S.E. from the large gates leading out of the park. The second aperture B is situated rather more than 600 yards distant from A, in a direction S.W. by W., while the third, C, is only about 110 yards from A, in a south-easterly direction. The subsidence A, being filled up, is not available for examination; but the committee have examined the fall C, with very interesting results. The subsidence was found to consist in the first place of a shaft, almost circular in form, being 7ft. 8in. in the longest diameter, and 6ft. 9in. in the shortest. The sides went down vertically to a depth of 18ft., and had all the appearance of a well, or artificial shaft. At the bottom was a heap of fallen earth, and when this was removed the sides were found to recede, the hole increasing in its diameter to about 14ft."

↑ The subsidence map updated by T.V. Holmes in 1907 with the line of the great sewer constructed under the heath; the sewer contractor provided important information about natural caves discovered in the chalk 100 feet below ground (see below). Click for a larger version (113k 1000 x 1053)

The Blackheath Subsidences Committee proposed many theories, mostly focussing on the action of natural groundwater, although some suggested collapsing denehole shafts. No firm conclusion was reached, but they were favourably disposed to the "hydro-geological" theory. T.V. Holmes curiously went on to state in a 1907 article that he thought the denehole theory was the best idea. He rejected the possibility of shallow mine galleries, despite the fact that sand mining had taken place on the heath in living memory at the time he was writing:

That a gallery was run in from one of the valleys in the side of the heath, or even from one of the pits which are to be found there, appears extremely unlikely.

A slightly bizarre illustration to show the cross section and scale of the subsidences appears in the Engineer article: a Victorian lady subsiding into the ground. What readers thought of this treatment is not known. The scale of the lady is woefully inaccurate; if the hole was 18ft. deep, that would make her about 20ft. tall.

↑ Click for a larger version (259k 1000 x 967)

Caves under the heath

Three very substantial natural caves in the chalk were encountered during the construction of the great sewer across the heath around 1903-4. Natural passageway was found near shafts 4 & 6 on the heath, as well as near the Mycenae Road shaft (see the plan of the heath further up this page for the line of the sewer). Bearing in mind that this was just one transect across the heath, it's safe to say that there must be dozens of similar cavities, some with active stream systems and others now dry (what cavers call "fossil" passageway). Chalk is a soluble rock, meaning that for the foreseeable future these caves will pose a hazard to human activity on and around the heath. The most likely cause for the majority of the Blackheath subsidences is voids from collapsed chalk caves migrating to the surface. The proven history of sand mining on the heath may also contribute occasional ground collapses due to the roof failure of shallow mine galleries; these collapses may be difficult to distinguish from voids that have migrated up from a much deeper level.

↑ Natural caves discovered under the heath; the diameter of the sewer is about 10 feet, which shows the scale of the cavities very well. Click for a larger version (2.28MB 3064 x 1090)

Blackheath subsidences historical round-up:

- November 12th (approx) 1798: "A singular accident happened last week at Blackheath. As a farmer and his son were conversing together in a field where a horse was feeding, on a sudden the animal sunk into the earth (hind feet first) to the depth of 15 feet, out of which he was dug, crushed to death. The cavity was only just sufficient to admit his body, the surrounding soil remaining firm."

- Transactions of the Geological Society of London Vol 4 1817: "In the year 1803, an extensive excavation which had formerly been made into these strata was laid open; it was supposed to extend to the chalk beneath, but the roof fell in and the passage became choaked [sic] up before it had been explored".

- John Winn (reminiscence given to Hart): "...about the year 1820 another place, not far from the large windmill near Whitfield's Mount, dropped down in the like manner to a great depth."

- [1876] "a strange subsidence occurred five years ago in Kidbrook Park Road, on the eastern border of Blackheath" Source: Popular Science Review, 1881, New series VOLUME V (volume XX of entire series)

- Thursday April 12 1878 - subsidence A

- Early November 1878 - subsidence B

- November 19 1878 - subsidence C

- F.C.J. Spurrell mentions in 1881 (Archaeological Journal XXXVIII) that other similar holes on the heath had been "behaving in a similar way; one of them was near "Washerwoman's Hole" [part of the heath adjacent to Royal Parade]. Some have been slipping gently for many years in the gravel pit near Whitfield's mound."

Blackheath Vale sand mine

Blackheath Vale, formerly a modestly-sized gravel pit, boasted its own mini-sand mine, one gallery of which was open as recently as the late 1940s/early 1950s. Two witnesses - Bob Land and another gentleman - both independently remember the sand "cave". Their account appears in a brief correspondence in the Blackheath Bugle blog of January 5th 2009, and I reproduce the excerpt here with the kind permissions of both the Bugle and Bob Land:

Bob Land
March 25, 2009 at 4:59 pm

Peter,
Regards the ”bomb dump” in Blackheath Vale , I remember that as well, do you remember seeing a ”cave” in the sand , about half up the sheer wall on the Heath side of the Vale, we used to venture into it, but did not dare to go more than one or yards inside, afraid that the roof of the cave might collapse.

regards

Bob Land

--------------------

Peter C
March 26, 2009 at 12:28 pm

I do remember the sand cave. It was actually at ground level. It went in about 20 ft. (give or take because I was little at the time!) and then turned back on itself a few feet away. I seem to remember that you carried on and came out alongside the entrance but memories are hazy now. I do remember Miss Collins (the head mistress of All Saints school) telling us that it was a dangerous place.

↑Click for larger version (414k 1024 x 621)
Whether this mini-mine was contemporaneous with the late 18th century sand mine at Whitfield's Mount is unknown. It would be easy to write off the Blackheath Vale tunnel as the work of children or late Victorian amateurs, but the tradition of mining sand a stone's throw away as well as the substantial 20 feet of passageway suggest something more premeditated.

Could Goffers Road have mine galleries underneath it?

Yes - both at Blackheath Vale and up the road at Whitfield's Mount. Lewisham Council please take note.

But how can there be mine gallery below the level of both ponds (Hare & Billet and Whitfield's Mount)?

Because a water-resistant horizon (probably clayey) puddles the ponds that occur on the gravel. In Blackheath Vale you are walking around well below the level of the Hare & Billet pond. The same effect would have occurred at the Whitfield's Mount mine.

What action should Lewisham Council take?

Goffers Road is already partially protected from heavy vehicles by the width barrier at the top of the village. Installing temporary 7 feet width barriers at both the Blackheath Tea Hut and Hare & Billet entrances to Goffers Road would be prudent, pending a thorough investigation using traditional techniques like boreholes (drilled, not percussion method).

Winn's Chalk Mine, Blackheath Hill

F.H. Hart, author of History of Lee and its neighbourhood (1892) gives us an account of a second chalk mine (ie NOT the Blackheath Cavern) at Blackheath Hill:

Mr John Winn, who then kept the Sun Inn, at the hill, informed us that about the year 1800 the earth dropped into an arched tunnel of chalk, north of the turnpike road, opposite these caverns, and that he and others went a great distance under the road, towards the Heath

Another account of the same date, extracted from the minutes of the New Cross Turnpike Trust Oct. 28th 1797, states:

It being stated to the meeting that within a few days past a great part of the Earth underneath the Road leading from Blackheath to the Limekilns on the North side had given way and with a part of the Road had fallen in a very considerable depth and that from the appearance of it more was likely to fall in owing to a large Cavern or Excavation beneath the Road, whereupon several of the trustees present went to the spot to view it and having seen the same and made Enquiry of some persons who were said to have explored the Excavation, it was proposed that a shaft or well should be dug in some part of it and the Cavity filled up with the earth from the adjoining Bank and be rammed in both perpendicularly and horizontally so as to make the whole solid and secure, but Mr. Driver and General Davis, two of the trustees present, offering to explore the excavation on Monday next, it was ordered that if upon Examination the plan proposed shall appear to be the most effective that it be carried into Execution, but at all events that immediate care be taken to prevent any accidents happening from the falling in of the Road and the Meeting agreed to adjourn...

Nov. 4th, 1797 General Davis and Mr. Driver reported that they had examined the Excavation under the Road leading from the Limekilns to Blackheath, that it runs in a southern Direction 33 ft. then turns south-west 33 feet 6 inches, then to the north north-west 47 feet to the centre of a part which rises higher than the rest in the form of a dome; that the average width is from 15 to 19 feet and about 12 feet on an average in height. They also reported that a shaft had been sunk at the Dome where the Earth was not more than 8 feet to the surface of the Road¹

↑ Reconstruction of the 1797 account; this is obviously not an isolated section of passageway, but part of a larger mine (click for a slightly larger version)

Remember that by 1800 the Blackheath Cavern had already been open for 20 years, and was being visited/used daily - there can be no doubt that what opened up was a different mine.

"North" of the turnpike road (Blackheath Hill) means on the same side as the Blackheath Cavern, although "opposite" implies it was on the other side of the road, which is puzzling. In any event, Winn had almost certainly been in the Blackheath Cavern, and well knew that the mine he entered via this ground collapse was not the same working. I suggested in my discussion of the Blackheath Cavern (here and here) that there is a missing mine in the vicinity of Maidenstone Hill; Winn's mine is a strong candidate.

Is Winn's mine the same as that explored by General Davis & Mr Driver? I'm not convinced it is. The survey by Davis and Driver can in no way be described as going "a great distance under the road, towards the heath".

Per von Scheibner was told a recollection that decades ago in the old pit on the South side of Blackheath Hill (bottom right of the aerial photo) there were blocked tunnel entrances; it's unclear what that reminiscence referred to.

↑ Blackheath Hill/Maidenstone Hill junction; click for a slightly larger version (373k 674 x 611)

This excellent 1823 map shows the area in question, but is of only limited help in unravelling the "north of the turnpike road, opposite these caverns" puzzle:
↑ Click for a larger version (483k 906 x 686)

What is most worrying is Winn's assertion (unique among the many historic reminiscences of the cavern) that "he and others went a great distance under the road, towards the Heath" - if he is referring to Blackheath Hill, then it opens the door to the possibility of more undiscovered galleries under the road in addition to the small one found after the 2002 collapse that simply connected open pits on each side of the road.

The investigation carried out after the 2002 roadway collapse is turning out to have been completely inadequate to address the issue of mining activity on Blackheath Hill. This inadequacy was made worse by the lack of subterranean exploration by mining historians - an experienced pair of eyes underground is everything. The agencies involved need to discuss with us a fresh schedule of investigations using proper techniques such as sinking shafts and driving headings underground (as was done at Plumstead in the 1950s) to intercept galleries. Keyhole surgery may be a good technique for humans, but for investigating chalk mines it's worse than useless.

¹ This information was quoted in the Records of the Chelsea Speleological Society, but unfortunately it was not quoted verbatim, leading to loss of accuracy in some details.

Whitfield's Mount Sand Mine, Blackheath

It is absolutely certain that a sand mine once existed in Marr's Ravine, a long gravel pit located between Whitfield's Mount and Hare & Billet Pond. F.H. Hart, author of History of Lee and its neighbourhood (1892) personally inspected the tunnels:

The truth of this statement can be vouched for by the writer, he being one that entered this cavern at the end of the gravel pit, near the pond at the Mount ; it was then shown by a man from Lee, who charged 3d each for the use of candles. This cavern has fallen in on several occasions since that date, within the past 60 years, which the writer knows from experience.

Hart's location at the end of the gravel pit, near the pond at the Mount must refer to Marr's Ravine, an old pit that stretched down from the Mount to Hare and Billet pond; here it is, pictured in the 1890s or perhaps 1900-ish (it was filled and levelled in about 1903 with spoil from sewer digging) looking back towards the Hare and Billet:

↑ Click for a larger version (1.71MB 2000 x 1205)
A mine working on the heath was exposed by a subsidence in 1803 (see the note below from the Transactions of the Geological Society of London Vol 4 1817) - this is presumably the same one that Hart entered some years later, although it could have been elsewhere on the heath. Could the galleries have gone down to the chalk as suggested in the Geological Society excerpt? Highly unlikely, because when digging into the heath, copious quantities of water are encountered about 12m down, where it runs along the top of an impervious stratum, which would tend to inundate galleries dug below it. The chalk occurs much deeper at about about 30-35m.

↑ Note the description of an "extensive excavation". The Geological Society of London was not known for exaggeration but rather for stating unadorned scientific facts.

F.C.J. Spurrell also comments in 1881 (Archaeological Journal XXXVIII) - quoted in context with most relevant words in yellow:

The Times and local prints contain particulars of holes dropping in recently on Blackheath. They are discussed by Mr. T. V. Holmes and Mr. Lemon; three of them being partly visible in 1879. I can remember others, however, on the heath behaving in a similar way; one of them was near "Washerwoman's Hole" [part of the heath adjacent to Royal Parade]. Some have been slipping gently for many years in the gravel pit near Whitfield's mound

↑ The pit in 1805... 

↑ Hart's location of the mine

In recent decades this mine has been very quiet, but nevertheless I would recommend that a closely-spaced borehole investigation is carried out in the pit and around its perimeter to ensure public safety; each bore should go down until the water-on-impervious horizon is encountered (typically about 12m across many areas of the heath).

↑ Click for a larger version (825k 1024 x 888)