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The investigation of the site forms a component of a long-term project into the history of iron-making around the outcrop of the iron oxide ores of Glamorgan border vale.
The Caergwanaf uchaf has been investigated over a number of years, with many research questions left to answer.
The site is located on the west side of the Elai (River Ely), on the low hill east of Caergwanaf uchaf and the river floodplain below. To the east side of the Elai at this point was part of Clun Park, demesnes land of the Lordship of Miskin and a medieval deer park. The Caergwanaf ridge forms the route of the old coach road from Llantrisant to Pendoylan, which is met at Caergwanaf uchaf by a minor road from the east, skirting the south-eastern margin of Clun Park and crossing the Elai by a small bridge near New Mill (usually now referred to as Miskin Bridge, but formerly known, from at least the late 17th century, as Pont Felin Newydd). An earlier crossing on this site is indicated by references in 1536/9 to Pont Risclidog of wood (Leland, Itinerary), in 1578 to the bridge of Ridisklidoge, decayed or of Rhyd Sglydog (Rice Merrick, Morganiae Archaiographia) and in 1588 to Pont Rydsklidog (Pembroke Survey of Miskin; NLW Bute MS 2361) as the southern corner of Clun Park. The ford in existence in the middle of the nineteenth century lay to the north of the bridge at New Mill, but its western approach was used as dump for construction waste which now forms a conical "tump" to the northwest of the bridge.
In the 17th century the boundary of the park was described (demesnes survey of 1666; NLW Bute Box 104/4) as being along "the highway leading from Cornel y Park toward Cowbridge unto a gate near the house of James ap John, smith and from thence to the river of Ely". This line is probably that extant on the park map of 1824 (GRO D/DD BE1/2), on which the boundary passes by the site of the smithy recorded on the 19th century Ordnance Survey maps, running along the northern margin of the property now the Miskin Arms Public House, reaching the Elai about 25m above the railway bridge (220m above the road bridge). The "gate" might have been on the line of the present Miskin - Pontyclun road. It seems unlikely that these two descriptions refer to the same position of the SW corner of the park, but rather that the Cowbridge road crossed the Elai close to the position of the present road bridge, and that the corner of the park was detached to make provision for the mill race for the New Mill (Felin newydd) in the early 17th century, and which appears to have originated just above the railway bridge.
Caergwanaf uchaf is situated on the east side of a small valley running northward to the Elai. To the west of the small valley the ground is higher, before rapidly falling to west into the valley of Nant Dyfrgi, which separates the area of Caergwanaf from Talygarn to the west. The geology of the site is quite complex. The old N-S road past Caergwanaf uchaf Farm is running approximately along the line of geological fault. To the west of the fault, the fields west of the farm lie on the middle part of the Carboniferous Limestone, but our site lies on the lower part of the Carboniferous Limestone, with the underlying Devonian rocks occurring towards the M4. The Carboniferous Limestone is overlain by boulder clay on the hilltop, and by alluvium in the valley.
Surface finds of slag, together with old accounts of a large slag dump previously visible near the river "100 yds upstream of Miskin Bridge" (Strahan & Cantrill, 1904), indicated that the site was a bloomery iron smelting site of considerable significance. The field on the floodplain to the north contains traces of what appears to be a pond, probably fed by springs in the valley side above. The site was undated, but the volume and type of slag, together with the apparent use of water power, initially suggested a late Medieval to early post-Medieval age (15th-16th centuries). There are three suspected 16th century bloomeries (Mwyndy [ST 057815], Pont-y-parc [ST 050823] and Rhiwsaeson [ST 071827]) in the area around the iron ore outcrops of the belt between Cefnyrhendy and Llwyn-saer (subsequently referred to more simply as the Miskin iron ore bodies, to differentiate them from the Llanharry bodies to the west and the Lesser Garth bodies to the east). A substantial body of documentary evidence exists concerning ironmaking in the area in the 1530s and 1540s. Just across the Ely, 400m to the north of the Caergwanaf site, is an early industrial complex at Hendy Isaf [ST 044812]; this may be a finery forge associated with a short-lived blast furnace built in the area to north, now called Talbot Green, in the 1590s.
Despite the initial expectation that the Caergwanaf site would be post-medieval, all the evidence now points to the complex being entirely Roman in age.
The main ore sources of the Clun Park area have been worked on several occasions. A small dump of bloomery and bloomsmithing waste of probable late Roman age was found close to School Lane (Young & Macdonald 1998). This site also yielded a few sherds of what may be 11th-12th century pottery. Little is known of medieval iron making in the area, although it has been suggested (Young & Thomas 1998, 1999; Redknap & Young 1998) that the cargo of the Magor Pill boat (13th century) may have come from this part of Glamorgan. The episode of exploitation in the 1530s and 40s is well attested by documentary evidence. There was a short-lived blast furnace in the area in the 1590s (see above; Davies 1995), but there is no evidence that this employed ore from Miskin, and subsequently there is little evidence for mining until the 1850s.
The Bute Mine worked from 1854 to 1880, in ground to the west of the Llantrisant Road. Production figures only exist from 1859, after which 318,000 tons of ore were raised. The mine comprised two opencasts, divided by a ridge which carried the road and tramway (these were moved to allow most of this area to be quarried too). Drifts worked deeper levels from the base of each pit. Earlier workings, to which were then attributed a Roman age, were encountered during the opening of the mine in 1854/5. Mwyndy Mine worked from 1855 to 1854, in ground to the east of the Llantrisant Road with a post-1859 production of 1,153,000 tons. The mine was extensive, with the flooded opencast to the west, with drifts extending down from its base. To the east, further opencast works were supplemented by both shafts and drift openings. The ground of Cefn-yr-hendy was also worked by the Hendy Mine just after the First World War, which was dug through from a site (now Pontyclun Rugby Club) on low ground to the west, eventually reaching the former Bute Mine. Little was found, with most ore pockets discovered having been previously worked-out,
The section provided by Vivian (1885) serves to illustrate the geology in the area of the flooded opencast. The mineralised belt is where the unconformable contact between the Dinantian limestones (the local highest unit is the Hunts Bay Oolite Group) and Namurian shales is overlapped by the unconformable base of the Triassic. The mineralisation generally lies within the top of the Carboniferous limestone, capped by the Namurian, but flattens southwards where the limestone had been capped by the Triassic. Ore bodies also extended southwards for up to 150m along "fissures" in the limestone and in the area of the pit, an ore body extended vertically downwards from the unconformity, parallel to strike. Typical ore lithologies are brown ores, dominated by goethite, which are often highly silicified. The silicification often forms a distinct layering (as also seen at Llanharry and in the Magor Pill boat cargo). Some very fine grained siliceous red ores are also present, and a silicified yellow dolomite is also common. The cavernous features within which the ore was emplaced may have been generated, at least in part, by karstic activity of Triassic age. The extension of the ores upwards into the Triassic conglomerates gives some indication of age.