Summary Heat flow is estimated to be 2.19 μcal cm−2s−1 at Rookhope, northern Pennines, and 20 km SE it is 2.29 μcal cm−2 s−1 at Woodland. Heat flow at South Hetton in the Durham coalfield has been re‐computed from 19th century observations to be 1. 38 μcal cm−2 s−1. The northern Pennine values are about twice the value of about 1.16 μcal cm−2 s−1 at Kirkleatham and Tocketts in North Yorkshire. The high heat flow at Rookhope may be explained in part by the high observed radioactivity of the underlying Weardale granite, but even allowing for uniform radioactivity down to 9 km the heat flow contribution from beneath this depth appears to be greater below the northern Pennines than below North Yorkshire. The high heat flow at Woodland, which does not overlie the Weardale granite, is problematical but may be related to rise of hydrothermal water near the Butterknowle fault system.

A deep borehole drilled at Rookhope in Weardale, Co. Durham in 1960–61 proved the Weardale Granite below Carboniferous sediments at a depth of 390 m; an account of the Carboniferous rocks is presented. The boring commenced in the basal Namurian Great Limestone and entered the Dinantian at the bottom of the limestone at 25 m. Most of the ten Brigantian cyclothems sectioned in the borehole are compound and average 30 m thick. They consist of an initial cycle often more than 20 m thick and overlying minor cycles that are usuallyless than 6 m thick. Each of the cycles, both initial and minor, are coarsening upwards sequences from marine limestone or mudstohe to deltaic and fluvial mudstone, sandstone, seatearth and occasionally a thin coal seam. The underlying Asbian consists of seven cyclothems averaging 8 m thick. By contrast they are simple coarsening upwards sequences similar to the Brigantian initial cycles, but no minor cycles are developed.Marine fossils are abundant in the majority of cyclothems and are listed against their stratigraphical horizon in the borehole. Special studies of miospores, conodonts and foraminiferans from the borehole are presented. A consensus of biostratigraphical evidence shows that the base of the Carboniferous succession in the borehole is of early Asbian age.

The geological results of a borehole drilled to investigate the coincidence of centres of a zonal pattern of mineralization ( Dunham 1934 ) with areas of strong negative Bouguer anomaly are presented. The presence of a granite batholith with cupolas, postulated by Bott & Masson-Smith (1957) has been proved; the top surface of the granite was found at 1281ft depth and the boring was continued to 2650 ft in granite. The section begins in the Great Limestone at the base of the Namurian; a normal succession in the Middle Limestone Group (Lower Carboniferous) is revealed, the Smiddy Limestone with Girvanella Band being reached at 1056 ft 6in. The Lower Limestone Group differs from the section in Teesdale and the Pennine escarpment; in particular, beds equivalent to the Melmerby Scar Limestone, identified by their fauna of algae, corals, and brachiopods, have a rubbly lithology and are split by bands of seatearth, sandstone, and shale. Beneath them, clastic sediments 38 ft thick with marine fossils rest on the weathered surface of the granite. Two quartz-dolerite sheets were proved; the Little Whin sill, 6ft, is in the Three Yard Limestone as at Stanhope, while the Great Whin sill, 192ft 9in, lies beneath the Jew Limestone, stratigraphically lower than in Upper Weardale. The Weardale granite, carrying biotite and abundant muscovite, has a low-dipping foliation in the upper part, but this becomes less obvious below 2225ft depth. Preliminary X-ray studies indicate that low albite, orthoclase, maximum microcline, and forms intermediate between the last two are present. Analyses of representative rocks for major and trace elements are given. Xenoliths are conspicuously absent, but aplites and pegmatites are common. The mineralogical and chemical effects of weathering before Carboniferous sedimentation, and of the mineralization, are described. The borehole was sited near the crossing of the Boltsburn lead vein (ene-wsw) and the Red fluorite-iron vein (ese-wnw), both of which dip towards it. Strong metasomatic mineralization, including green fluorite, blende, and quartz occurs in the Tynebottom, Jew, and Lower Little limestones. Numerous small veins were also cut both in the sediments and the granite. Though chalcopyrite is present against the lower sediments, there is no evidence of a concentrated copper zone. Fluorite continues into the granite, and pyrrhotine, not present above, is found from 1355 ft depth. Mineralization continues to much greater depths than have previously been proved in the area; its source presumably lies beneath the granite cupolas, which may, however, have guided the rising fluids.

SUMMARY The Northumberland Trough has been recognised as a major east-west structural unit across northern England for almost a hundred years. Lying between the Northern Pennine Block to the south and the Southern Uplands of Scotland ridge to the north, the Trough stretches across Britain from the North Sea to the Solway Firth. Two basins have been described within the Trough – the Northumberland Basin in the east and the Solway Basin in the west. The basins are believed to be separated by a shelf, with shallow basement running northwards and continuing the line of the Pennine ridge across north-east Cumbria. The Bewcastle anticline is regarded as a drape over this basement high. The oldest deposits proved in the basins are Chadian (Lower Carboniferous), with no confirmed record of the lowest Courceyan Stage. A complete sequence of more than 3 km of Dinantian and Silesian sediments is known, ranging up to late Westphalian D. Strong differential subsidence in the Northumberland Basin came to an end in the Namurian, and the Westphalian was laid down under conditions of uniform epeirogenic downwarping that effected the marginal blocks in the same way as the Trough. The Solway Basin had very similar Dinantian deposition, but subsidence was more variable in the Silesian and persisted longer, at least until the late Westphalian. Shallow marine and deltaic sedimentation characterise deposition in both basins, with subsidence and rate of deposition having been approximately equal. Marine transgressions and deltaic regressions were a major feature of deposition and caused the development of well-ordered sedimentary cycles of coalbelt type. Four phases of trough filling can be recognised: 1. Progressive marine transgression. 2. Gradual marine regression. 3. Upper delta plain sedimentation. 4. Emergence, with lacustrine sedimentation.

Summary Geophysical evidence has provided an explanation for the disposition of massif and basinal structural regions in northern England. The massifs are underlain by granite masses of Caledonian age or older which have exerted a strong positive tectonic control since their emplacement in Devonian or earlier times. These positive structural regions formed gradually dwindling islands and uplands during Carboniferous times at least until the late Viséan and in some cases considerably later. Structural basins and troughs surrounded the island areas and here great thicknesses of sediments were deposited. Deposition began in Tournaisian times and in some regions was a continuation of Old Red Sandstone sedimentation. Subsidence, forming basins of deposition and causing the initial Carboniferous marine transgression, is shown to be of tectonic origin. Early basin formation and later, and more widespread, epeirogenic subsidence are linked to isostatic uplift of the Caledonian mountain ranges following the mantle flow theory (Bott, 1964a). Throughout the Carboniferous Period the Basement exerted a profound tectonic control on sedimentation. The distribution of sedimentary facies in basinal and cratonic regions is dependent on this control as is the continuous supply of clastic sediment. It forms the controlling mechanism of cyclic sedimentation of Yoredale and to some extent Coal Measures facies. The disposition of some Carboniferous knoll reefs may also have a basement control.