Corallian Building Stones – Upper Jurassic Limestones (Lead authors: WGT and PJB)
The Corallian Group of Formations was deposited in shallow seas 163 to 155 million years ago (Oxfordian-Early Kimmeridgian). In North Dorset there are four mapped formations (Fm), from the bottom up: Hazelbury Fm, Stour Fm, Clavellata Fm and Sandsfoot Fm. Total thickness is 40-100m. In South Dorset the equivalent five formations are: Nothe Fm, Osmington Oolite Fm, Clavellata Fm, Sandsfoot Fm and Abbotsbury Ironstone Fm. Total thickness is 65-160 metres.
In North Dorset a cross-bedded oolite, the Todber Freestone Member of the Stour Fm (equivalent to the Osmington Oolite), is actively worked in the Todber-Marnhull-Whiteway area, along with the overlying shell-rich Clavellata limestones. The latter vary from shelly oolitic grainstones (Shelly Clavellata) to sponge-spicule lime mudstones (Spicular Clavellata). A range of these rock types occurs in buildings in North Dorset (e.g. Blandford Forum, qv). In South Dorset the Corallian has been used as a building stone in the Abbotsbury area but, until very recently, quarrying had ceased (the Ilchester estate has re-opened a quarry near Oddens Wood, Abbotsbury).
Shelly Clavellata (WGT)
Late Oxfordian limestones (3-12m), with the common bivalve Myophorella (formerly Trigonia) Clavellata), overlie the Middle Oxfordian Stour Fm after a time gap of maybe 0.5 million years. In North Dorset, the contact can be undulating with limestone pebbles eroded from the underlying Todber Freestone Mbr. The Shelly Clavellata is a bioclastic quartzose (c.5%) ooidal grainstone. Sparite-filled Myophorella and gastropod moulds are characteristic.
Spicular Clavellata (PJB)
The fine-grained Spicular Clavellata is easily recognised as a yellowish micritic mudstone rich in microscopic casts of Rhaxella sponge spicules (kidney-shaped), a contrast to the generally more shelly blue to grey sparitic Shelly Clavellata. For a short period during deposition of the Clavellata Fm, south of Sturminster Newton and into Hazlebury Bryan, there was a low-energy area of lime mud with the sponge Rhaxella, deposited contemporaneously with the ooidal shoals immediately to the North.
At Woolland Chapel (qv) and the Blandford Cemetery chapels (qv) both these building stones were used together in coursed rubble exterior walls. The Spicular Clavellata deposits are not commonly used in buildings, so to identify them it was necessary to make thin sections from frosted-off detritus.
[see “How we study building stones” – this website, Building Stones section]
A historic diary records the external re-cladding of Woolland Chapel from the Rivers family estate pits at Hazlebury Bryan. Consecration in 1856 was within a year of completion of the two chapels at Blandford Cemetery, built from the same matching Spicular and Shelly Clavellata. Both these Clavellatas were available at this time about eight miles upstream of the Stour in Newton Village South Quarry, adjacent to Common Lane.
Todber Oolite (Marnhull Stone) (WGT)
The Todber Freestone (up to 5m) is a cross-bedded fine to coarse ooidal grainstone with a low to medium shell content, virtually no quartz grains and with an intergranular sparite cement. Some sparite is likely recrystallized lime mud (micrite), thus grainstone-packstone. Some bioturbation. Low percentage of iron minerals gives buff-orange colour when oxidised. Very similar to Osmington Oolite.
The Todber Freestone is currently quarried in the Todber-Marnhull-Whiteway area of NW Dorset and is widely used in modern stone-clad buildings (“Marnhull Stone”). See Whiteway Quarry (“Quarries & Pits” this website).
Osmington Oolite Formation (WGT/JT)
The Corallian Group (65m) of the Dorset coast area comprises a range of clastic and carbonate lithologies. The Osmington Oolite Formation (30-40m) contains the best building stones within the Group and has been quarried extensively and widely used in buildings in the Abbotsbury area. This shelly bioclastic oolitic grainstone can be pale cream to orange-brown depending on the iron content.
There are several beds within this Formation, some only thick enough to be used as rubble, or cut stone, but some, deeper in the Formation, being thick enough to be used as ashlar. The weathered appearance of the stone in historic buildings produces a darker orange colour than the Todber Freestone oolite used in North Dorset. It also contains more fossils than the Todber stone.
Abbotsbury Ironstone Formation (WGT)
This is a local development of ferruginous (c.30% Fe) quartzose sandstone, up to 57m thick, at the top of the Corallian Group immediately below the Kimmeridge Clay Formation. It is thickest at Abbotsbury and was extracted as iron ore during WW I, however the high silica content prevented large scale development. Its distinctive rich brown colour, quartz grains and rusted ooids are diagnostic. It is easily recognised in buildings in the Abbotsbury area.