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Stone Roofing in England Terry Hughes.
Since the earliest days of the history of the science of geology the Welsh Borderland has attracted the attention of geologists by the great variety and interest of its formations, for in no other area can the sequence of the Palaeozoic rocks be seen to such advantage and in such a comparatively small district. (Earp and Hains 1971, 1)
Unlike other stone slate regions of England, the Welsh Marches have not been defined by their geology for this study. Geographically, the stone slate usage extends from the Bristol Channel to Shropshire. For this study it is defined to the west by the Welsh border, and to the east roughly by a line due north from Gloucester. For convenience, the Pennant stone of the Bristol region has been included with the Pennant of the Forest of Dean and South Wales.
The region cuts across the grain of the geological succession from the Ordovician to the Jurassic; a period of about 300 million years. This makes for interesting roofs but difficult research. Because the geology changes rapidly over short distances, and because so many different stones have been used for roofing, this section is inevitably incomplete. There is no doubt that many small delves, hidden away in forests, in deep narrow valleys or on remote hill tops, still remain to be discovered. Similarly, there are many sources of stone slates from the same formations across the border in Wales. These have not been included in this report but they should not be overlooked when searching for potential new supplies. From Llandeilo in South Wales to Ludlow the lowest part of the Silurian Pridoli Series consists of micaceous flaggy sandstones, known as the Tilestone Formation. (Today it is more correctly the Long Quarry Beds in the south and the Downton Castle Sandstone Formation in the north.) Its use is differentiated by the initial capital in ‘tilestone’, otherwise used to describe any stone slate. 
An example of the difficulty in determining the historical provenance of stone slate roofs in the region is provided by Ashleworth Tithe Barn, north of Gloucester, which was to be re-roofed during 2001 (Figs 60 and 61). The barn is thought to have been built between 1481 and 1515 but the roof was reslated in 1885 and again in the 1940s. In 1942, Harold Trew, a Gloucester architect, reported that the roof was covered with ‘Cotswold stone tiles’ (unpublished letter to the Society for the Protection of Ancient Buildings, 1942). In fact, the roof today includes red and grey-green Old Red Sandstone and Forest Marble slates but it is not possible to determine which is the earliest or even if they were both installed at the same time. The Old Red Sandstone is only six or seven miles away at Newent to the west or the same stone could have been brought up the Severn from the Forest of Dean area. The Forest Marble would have come overland from the Cotswold Hills fifteen miles to the east.
To further complicate matters Ashleworth actually stands on rocks of Jurassic-Lower Lias age and field walls in the vicinity include thin stone which would be suitable, and therefore might have been used in the past, for roofing (Fig 62). Stone for field walls was never carried far. Indeed, stone walls often only exist because they are a ‘waste product’ of a nearby quarry which was primarily worked for more valuable products such as the Lias masonry of Ashleworth barn. The convenience of a nearby supply of Lias roofing might well have been the overriding factor in deciding what was used on the roof originally. (Sources of lias stone are discussed below.)
So, at present, there is a mixture of stone slates and an indeterminate history of other stones from an earlier date. In similar situations elsewhere, commentators have added further confusion by incorrectly identifying sandstones as limestones and vice versa, because they have incomplete knowledge of all the possible, local options.