This book is concerned almost wholly with a diverse suite of carbonate rocks that were formed near the margins of shallow tropical seas during the last 5-7 million years of the Permian period (300-251 Ma). These unique rocks, collectively known as the Magnesian Limestone, have been studied for more than 160 years and the names of some of the early workers - Geinitz, Murchison, Phillips, Sedgwick, Sorby - would grace any geological hall of fame. Despite this formidable assault, and the efforts of a host of later workers, the Magnesian Limestone still retains many of its secrets. Permian marine rocks crop out on both sides of the Pennines, but those of the Zechstein Sea to the east are by far the thicker and more varied, and in these lie all but one of the sites selected for special protection. Detailed accounts of the rocks in 26 such sites form about half of this book and the normal and special features of these sites are compared, contrasted and placed in their mutual context in the remainder of the book. The sites were selected according to a range of criteria, including uniqueness, representativeness, historical importance and suitability for teaching purposes and research; most are inland quarries but a few are in the unrivalled coastal cliffs of classical County Durham where the main difficulty lies in deciding what not to select.