The strata of the Purbeck Limestone Group

Caution: this page is a very tentative and incomplete attempt to reconcile the terminology of this very complex group of strata, as an aid to identification of stone used for ancient artefacts. It probably contains many errors, and readers should use it with caution !

The description is based on the type-locality, Durlston Bay, Swanage. Columns 1-3 refer to the numbering, description and thickness given by Clements 1993; column 4 to Bristow 1857 as reprinted in Damon 1884; column 5 to the specimens displayed at Langton Matravers Museum.

I have used the traditional but informal division into Upper, Middle and Lower Purbeck, but my more detailed stratigraphical terminology is basically that of Clements (1993), who recognises two Formations and fifteen Members within the Group. Beside this, I have also indicated the terms introduced by Westhead and Mather (1996), who would have only five Members, demoting Clements' fifteen units to the rank of Beds. While this newer scheme has definite advantages for correlation with other localities, I have hesitated to adopt it fully: for the word "bed" is often used to refer to still lesser units, e.g. the "Crocodile Bed" (Clements' DB221), or the "Unio bed" (DB239) (which is not part of what Westhead and Mather would call the Unio Bed), and confusion might follow.

In column 6, "S" means I have a sample from the type-locality; "S ?ident", uncertainty whether the sample is really from the bed with this DB number; "P" means that I have a photograph, "Pf" that the photo shows a fossil; numbers after a photo reference indicate film and frame. "Rockfalls" means that my study of the type-locality was impeded by fallen material. Numbers preceded by "WB" relate to Ensom's section of the Purbeck Beds at Worbarrow Bay.

Ellipses (...), or gaps in the numbered sequence of strata, indicate that beds have been omitted from this list. if Rockfalls are not mentioned, omitted beds are mostly shales or other soft material that are probably not of interest as workable stone. The main exception is the Cypris Freestones Member and below (Lower Purbeck), which includes important workable stones that are not well displayed at the type-locality, and must be studied at locations farther west.

For the appearance of some beds of the Purbeck Limestone see the website of WJ Haysom and Son and Lander's Quarries.
Alternatively try Archaeology Data Service: on the first page select "I agree", on the second "Stone in Archaeology interface", on the third "Enter", and then at last you find a list of rocks ...

The common fossils of the Purbeck Limestone group are gastropods, bivalves and ostracods; follow this link for details.

DURLSTON FORMATION

Upper Purbeck

Upper Purbeck = Peveril Point Member of Westhead and Mather. Photograph, general view

DB	Clements' description	metres	Bristow	Langton Mus.	Notes
Upper Cypris Clays and Shales Member
...		...
244b	Upper PM = Blue Marble; Viviparus limestone	0.76	91		S, Photo, 2002-04:5.18
...		...
241	Lower PM = Red Marble; Viviparus limestone	0.81	89	1a, Red PM	S, Photo, 2002-04:5.13
240
239	Unio Bed	0.30	87		S
238
237	Green Marble; Massive Viviparus biosparite	0.30	85	1b, Green PM	S, Photo, 2002-04:5.12
...	(shales etc.)	...	84
225	biosparite	0.15			S, P 2002-04:5.11
224	shales with thin limestones	0.53
Unio Member
223	Greenish biosparite	0.15	83		S, Pf. My photo 2002-04:5.14 shows a fossil found in the upper surface of this bed which is so far unidentified. Suggestions welcome.
222
221	complex, incl. Crocodile Bed, bottom limestone cemented to bed below	0.38-0.53	81-79		S, P 2002-04:5.10 upper surface of bed.
Broken Shell Limestone Member
220	Broken Shell Limestone; Massive shell-fragment biosparite	2.90	78, Soft Burr	2, Burr	S, `Soft Burr' is probably a wrong name due to confusion with the Dirt Bed `burrs', see Damon 1884 p.115 on the Purbeck beds in Portland. P 2002-04:5.9

Middle Purbeck, upper part

Middle Purbeck, upper part = Stair Hole Member of Westhead and Mather

DB	Clements' description	metres	Bristow	Langton Mus.	Notes
Chief Beef Member
219	Shales with beef	0.46	77		S, P 2002-04:5.8,22
218	Lt grey biosparite	0.23	76b		S, P 2002-04:5.20,22
...		...			Rockfalls
Corbula Member (part-marine)
189-8	Bivalve biosparites	0.16	70		Not seen
...		...			Rockfalls
184-3	Pinkish biospar/micrite, Toad's Eye (183)	0.36	68		?Not seen
...		...			Rockfalls
178	Pinkish cream shell biosparite	0.67			S ?ident., sandy thin-bed limest, surface crystals
...		...			Rockfalls
154	Rough-bedded bivalve biosparites and shales	0.66	59		S ?ident., 2 chips
Scallop Member (part-marine)
153-44			58, White Roach	3, Rag?	Rockfalls
Intermarine Member (Upper Building Stones)
145	shale	0.10
144-141	Laning Vein	2.20	56-57	4, Lannen Vein	3 S, P 2002-04:5.25
140e-b	shales and limestones	c.0.60
140a	Royal Limestone	0.30	55	5, Royal	S ?ident., cream shelly biosparite with green seam
...		...		6, Freestone?	Rockfalls
For a picture of a cliff section of the lower Intermarine and Cinder Members in Durlston Bay, follow this link
133	Red Rag	0.74-0.79	54, Red Rag	7, Rag Mead?	S, 2 chips, coarse bivalve biosparite
132	calcareous mudstone	0.30-0.25	53, Green shaly Marl
131	Under Rag	0.69-0.76	52, Under Rag		S, coarse bivalve biosparite
130	mudstone	0.48-0.43	51, shales 1'9''
			Freestone Vein 16':
129	calcareous mudstones	1.63	?50h-i, Shingle		Rockfalls
128	mudstone/micrite	0.23	?50g, Underpicking		Rockfalls
127-6	sandy, muddy, calcarenite biosparites	0.33	?50f Lower Shingle, rubbly limestone		Rockfalls
125	massive biosparite	0.79	?50e, Crab	?8, Grub	Rockfalls. Ensom 1994 citing El-Shahat and West 1983 identifies DB125-121 with the inland `Freestone Vein'
124c	`shaly' biospar/micrite	0.09			Rockfalls
124b	calcareous shale	0.05			Rockfalls
124a	rough bedded limestones; incl. bivalve biosparites	0.30	?50d, Roach, hard shell-limestone	?9, Roach	Rockfalls. Delair 1980 identifies `Pink Bed' of the Roach as Damon's bed 50d
123	massive biosparite with scattered sand-grains	0.23	?50c, Sugar bed	?10, Thornback	Sugar = Thornback acc. to Delair and Lander 1972; S, 2 chips, shell and shiny grains
122	Rubbly sandy biomicrite	0.18		?11, Whetson	S, 2 chips, shelly
121	Massive bivalve biosparite, weathers to `micrite'	0.48	?50b, Freestone, compact sandy grey limestone	?12, Freestone	S, crumbly, several bits, rough shelly texture; identified tentatively as the Freestone by El-Shahat and West 1983
120	Cream, thinly bedded calcarenites	0.41			S, chip, ?sand-grains
119	Dark grey ostracod shales grading into biosparites	0.43			S, chip, ?shelly
118	Massive looking hard limestone, thin laminae, ostracod seams weather proud	0.30	?50a, Blue bed		S, 3 small pieces
		5.45 total = c.18'	Bottom of Freestone Vein
117	calcareous clays	0.11
116b	Cream (bio)micrites	0.56	?49, Lias, 2'11"		WB123 = DB115-117, Ensom 2002b p.223
116a	Thin-bedded biospar/micrites	0.20-0.15	?48, Lias Rag, 2'4"		S, hand-size and smaller chip
115	Shales and clays	0.43	47 Dark sandy Marl 1'4"
114	Interbedded thin limestones and shales	0.94	46, Laper 3'0"	13, Laper	Downs Vein = Laper = Bristow 46 acc. to Delair and Lander 1972; S, cf. roofing-stone
113	Massive limestone with chert near top, crowded with small lamellibranchs	0.56	45d, Shell-limestone 2'9"	15, Downs Vein (14 Grey Bed missing)	Not Tombstone bed (Arkell 1944); Downs Vein (Salisbury 2002, Ensom 2002b p.223); S, shelly
112	Shales and limestones	0.23	45c-a, marl, limestone, shale 9"	15, Downs Vein?
Picture of cliff section of the lower Intermarine and Cinder Members in Durlston Bay, May 2002: large (300K) small (77K) © John Palmer, 2002-04:5.23 (east) and 2002-05:6.5 (west)
Cinder Member
111	Massive Praeexogyra distorta shell bank	2.95	44-43	16, Cinder	S, P 2002-04:5.23, 2002-05:6.4, 5

LULWORTH FORMATION

Middle Purbeck, lower part

Middle Purbeck, lower part = upper part of Worbarrow Tout Member of Westhead and Mather

DB	Clements' description	metres	Bristow	Langton Mus.	Notes
Cherty Freshwater Member (Lower Building Stones)
110	Dk grey shales	0.41			WB118 approx. (Ensom 2002b p.223)
109	Shales	0.038
108	Feather Bed, cream bivalve biosparite, Thalassinoides on base of bedding planes	0.61	42, Feather or Button	17, Button	Feather Bed, Salisbury 2002; S, taken from a bed adjacent to Thalassinoides
107	micrites grading softer downwards	0.14
106	Thin bedded biosparites	0.08
105	Calcareous mudstone	0.24
104	Calcareous clays	0.19
103	Massive limestone with chert near base, footprints on underside	0.25	36, Cap	18, Cap	Cap = DB103 acc. to Ensom 1986 and Ensom 2002b p.223. DB103 = WB117 acc. to Ensom 1994 (p.80 and fig.2). See note at end.
102	Shales with thin sandy biosparites at base	c.0.46	35, Sly	19, Sly	Sunnydown Sly bed, and approx. WB116: Ensom 1987 fig.1, Ensom 1988 corrected by Ensom 1989, Ensom 2002b p.223, Salisbury 2002
101	Cream weathering biospar/micrites, bivalves, gastropods	1.40	34	Here or below: 20, Five bed (or Six bed); 21, White; 22, Brassy; 23, Tompson (Tombstone)	New Vein acc. to Ensom 1987 fig.1, and to Salisbury 2002; S with shell-fragments
100	Gypsiferous shale	0.057
99	Muddy biosparite/micrite	0.057
98	Dark shale	0.09-0.20
97	Flint Bed; very hard biomicrites with ... chert nodules, silicified fossils	0.84	31		S, weathered, with chert; P 2002-05:6.3, 4 have this bed at beach level
96	Firm shales
95	V. hard, brittle micrite
...		...
92	dark grey/green shale with celestine	0.25	top 10" of 27		Correlation after Clements 1966; Fern Bed, Salisbury 2002; P 2002-05:6.2 shows this bed at bottom of cliff, LBS above
...		...
89	Firm/hard clays/micrites
88	Soft calcareous shales
87	Hard biosparite, micrite intraclasts		25
Marly Freshwater Member
86	clay/micrite		24		P 2002-05:6.1 shows this bed at bottom of cliff and up to DB111
85	clay/micrite
84	argillaceous micrite
83	Mammal Bed; a Dirt Bed; Carbonaceous, calcareous shale and clay		21
82	Micrite, biosparite below
81	Micrite/clay
...		...
75	Dark brown shale grading down to clay		20

Note to DB103: DB103 = WB117, so DB104-110 correlate only with the shaly WB118, as WB119 is the base of the Cinder Member. Where Ensom (1986, 1988) says DB103 = WB112 he is making a mistake, as explained in Ensom 1989. Ensom 2002b p.223 says DB102-110 = WB116-118.

Lower Purbeck

Lower Purbeck = lower part of Worbarrow Tout Member, and the whole of the Ridgeway and Mupe Bay Members of Westhead and Mather

DB	Clements' description	metres	Bristow	Notes
Soft Cockle Member
74	Hard to v. firm fossiliferous micrite		19
...		...
43	Clay with selenite and gypsum masses		13
Hard Cockle Member (Clements puts `?')
42	Laminated clay
...		...
34	Rough hard limestone
Ridgeway Member of Westhead and Mather:
Cypris Freestones Member (Clements puts `?')
33	Calcareous clay
...	(Micrites, calcareous clays, shales, sandstones)	...		These beds correlate with the building stones of Upwey, for instance
17	(Micrites and biosparites with argillaceous partings)
Mupe Bay Member of Westhead and Mather:
16	(Between clay and micrite)
...		...
11	Thin-bedded biomicrites and fossiliferous pelsparites
10	Broken ostracod biomicrites and micrites			Lowest bed visible at Durlston Bay (Clements)
Broken Beds Member (seen e.g. at Durdle Door)
Caps and Dirt Beds Member (seen e.g. at Tout Quarry, Portland)

There are many other descriptions or sections of the Purbeck beds, including
Fisher O 1856;
Hutchins ed.3 p.685;
Mason NJ 1984