The geological formation of Cumbria falls into a roughly concentric pattern. The oldest rocks are found at the centre, forming the rugged scenery of the Lake District. Surrounding this centre and dipping away from it in all directions one encounters successively younger strata. The ancient core comprises an immense variety of rock types of Lower Palaeozoic age, including extrusive lavas, ashes, and other volcanic material, as well as great thicknesses of sedimentary slates, shales, sandstones, and limestones. Intruded into these rocks are numerous igneous bodies including basalts, gabbros, and granites, such as are to be found at Ennerdale, Buttermere, Shap, Skiddaw, and Carrock Fell.
These Lower Palaeozoic igneous, metamorphic and sedimentary rocks constitute the greater part of the geology of Cumbria and are of enormous geological interest; but no examples of these rocks are represented as stone types in the present study and they need not be discussed further. The stones described in this volume were obtained entirely from sedimentary rocks of Carboniferous, Permian and Triassic age surrounding the Lake District. The following notes describe the location of outcrops, with brief petrographic descriptions of each rock type.
Lower Palaeozoic rocks are surrounded unconformably by rocks of Lower Carboniferous age on all but the western margin. This unconformity is often marked by the presence of a conglomerate which is particularly well developed in the area of Mell Fell and from there south to Tebay. Next in the succession, the rocks of Lower Carboniferous age (Dinantian) are predominantly marine deposited limestones with lesser amounts of mudstones and sandstones. The succeeding Upper Carboniferous, the Millstone Grit (Namurian) and the Coal Measures (Westphalian), are mainly represented by sandstones, siltstones, and mudstones, with of course in the latter series, coal seams.
Most locations in rocks of Carboniferous age yield sandstone suitable for building and monumental purposes. Such sandstones are characteristically buff coloured, though variations from white to red can be found. Grain size varies from coarse to fine, the grains mainly of quartz with lesser amounts of feldspars and some muscovite mica which may be abundant at some horizons. An interangular matrix of clay minerals, usually kaolinite and illite, can be recognized as a soft amorphus material between the grains. Cementation is usually by silica which contributes to the durable nature of the stone. The colour of the sandstones is accounted for by the presence of iron oxides and hydroxides.
The consistency of appearance of Carboniferous sandstones in hand specimen is such that it is not usually possible to assign the stones to a particular horizon or locality within the area of their occurrence. Microscopic analysis of thin sections, by which the nature and abundance of constituents can be ascertained, might lead to such conclusions, but obviously such a line of study was not possible in the present context.
Following the Carboniferous and lying unconformably upon it are rocks of Permian age. The Lower Permian is represented by the Penrith sandstone, a red sandstone derived from wind blown sands deposited in a desert environment. The present outcrop of the Penrith sandstone is almost entirely confined to the Vale of Eden, where it extends from Kirkby Stephen in the south to near Carlisle in the north. Around Appleby the sequence is interbedded with breccias (known locally as brockrams) consisting of angular fragments mainly derived from Carboniferous limestones.
Penrith sandstone has several noticeable characteristics, the most immediately obvious being its red colour. This colour is due to a thin coating around the grains of clay minerals rich in iron oxide. The grains themselves are mainly of well rounded and well sorted quartz with a smaller amount of feldspar crystals, the latter usually whitened by chemical alteration. Close observation of the texture of the rock will show many of the quartz grains to have well formed crystal faces. These are not an original feature but are secondarily deposited silica overgrowths. The rock is characteristically free from mica, this having been removed by the winnowing action of the wind in the original environment of deposition.
Although the outcrop of Penrith sandstone is widespread throughout the Vale of Eden, not all of it is suitable as a building material. The most durable building stone is obtained from the upper part of the formation, where the grains are well cemented. Elsewhere the stone can be friable or barely consolidated and as such useless for building or monumental purposes. While emphasizing the red colour of most of this formation, it should be noticed that there are some localities where the colour has been lost by bleaching, this being most apparent where incomplete cementation of the grains has allowed access to percolating water.
Small quarries are particularly common on the fells north of Penrith, from where stones were transported to other parts of the Vale of Eden less well endowed with suitable building material. There are many old quarries in the area north of Cliburn (NY 588249), and north of Kirkoswald (NY 555414) the river Eden has cut a deep gorge through the formation, producing numerous natural exposes. Other large stone quarries are found at Appleby.
Rocks of Upper Permian age which occur in Cumbria include magnesian limestone, evaporites (such as gypsum) and shales, none of which is suitable for building purposes. The next workable material in the succession is the Triassic St Bees sandstone (of the Sherwood sandstone Group), which lies conformably on the Permian St Bees shale. This sandstone is an important local building stone and has a widespread outcrop. It is present in the east in the Vale of Eden, from Maryport in the west to the Scottish border in the north, and in the south-west coastal strip from its type locality at St Bees Head to Barrow in Furness. Above the St Bees sandstone, and intercalated within it in the Carlisle area, occurs the formation known as Kirklinton sandstone.
Like the Penrith sandstone, the most obvious feature of St Bees sandstone is its red colour. There are, however, colour differences between the two (St Bees generally being a duller red than the Penrith), as well as differences in texture and composition, which allow them to be differentiated in the majority of cases. The St Bees sandstone is a water-lain deposit and commonly contains muscovite mica. The beds are often finely laminated and usually occur as thin flaggy beds although sometimes more massive beds a metre or so thick can occur. In general the grain size is fine and the grains (of quartz with a few per cent feldspar) tend to be angular, though some coarser, more rounded grains are encountered at some horizons.
The Kirklinton sandstone is a strongly cross-bedded, soft bright red sandstone, which is generally considered to be a wind-blown deposit like the Penrith sandstone. In fact the two can be very similar in appearance and might on occasion be confused. The Kirklinton sandstone is, however, too soft to be of use as a building stone, but it is not impossible that it could be used for small monumental items. It is not widely exposed at its type locality of Kirklinton nor in the vicinity of Rockcliffe and Netherby.
It will be evident from the foregoing that in most cases it is a simple matter to differentiate Carboniferous, Penrith and Triassic sandstone, but problems of recognition can occur, especially when the material in question is highly weathered and perhaps encrusted with dirt or lichens. A possible source of error is the colour. Material which might be expected to be highly coloured can sometimes be bleached and, conversely, Carboniferous sandstones can be reddened if the iron minerals in them become oxidized. As to composition, a small fragment of St Bees sandstone might not necessarily contain enough mica to be evident in hand specimen, and this might then be mistaken for a fine grained specimen of Penrith sandstone, which can be similar in colour, texture and composition. In the present study most of the specimens could be assigned to the relevant geological formation. Where uncertainty arose in recognizing Permian and Triassic material, the non-committal term 'Permo-triassic' has been used.
