Columnar-jointed tuffs (bentonites) are located below the Lower Carboniferous Tideswell Dale Sill, Derbyshire, in an abandoned quarry. There are three zones of prismatic joints, columnar joints (∼60 cm thick and mean column width of 4.1 cm) and massive, conchoidally fractured rock sequentially from the dolerite contact downwards. The rocks are very fine-grained (1–10 µm) and consist mainly of sanidine, interlayered illite/smectite, and hematite, with minor quartz, apatite, montmorillonite, anatase and detrital minerals. High K2O and Fe2O3, low Na2O and low MgO are interpreted as due to alteration of felsic volcanic ash. The bentonite was contact-metamorphosed by the sill to temperatures of c. 300 °C. The progression of fracture geometries, thermal considerations and application of theories of column formation indicate that columns formed downwards away from the sill, due to prograde contact metamorphism of the originally clay-rich bentonite. The formation of columnar fracture networks by shrinkage due to igneous heating may have implications for the effectiveness of bentonite as a barrier for disposal of high-level radioactive waste. Regional dimensions include documenting widespread felsic explosive volcanism and inferring a Triassic oxidation event from palaeomagnetic data.

柱状接缝的凝灰岩（膨润土）位于一个废弃采石场的德比郡低石炭纪的蒂德斯韦尔·戴尔·西尔下面。从白云石接触向下依次有三个棱柱节，柱状节（约60 cm厚，平均柱宽4.1 cm）和块状，贝壳状破裂的岩石区域。岩石的粒度非常细（1–10 µm），主要由桑定，层状伊利石/蒙脱石和赤铁矿组成，还含有少量石英，磷灰石，蒙脱石，锐钛矿和碎屑矿物。高K 2 O和Fe 2 O 3，低Na 2 O和低MgO被解释为是由于长石质火山灰的改变。膨润土是由门槛接触变质为的温度Ç。300℃。裂缝几何形状的发展，热学考虑和柱形成理论的应用表明，由于最初富含粘土的膨润土的接触接触变质作用，柱远离井坎而向下形成。由火成因的热收缩引起的柱状裂缝网络的形成可能对膨润土作为高放废物处置障碍的有效性产生影响。区域范围包括记录广泛的长统爆炸性火山活动，并根据古磁性数据推断出三叠纪的氧化事件。