The Magnesian Limestone of England forms a comparatively narrow outcrop stretching southwards for about 150 miles from South Shields to Nottingham. At either end the formation is fairly well known from the many quarries and sections opened up in the industrial areas of East Durham on the one hand and South Yorkshire and Nottinghamshire on the other. Away from these regions, however, our knowledge is scanty, and particularly so in the North Riding of Yorkshire, in the tract of country between the rivers Tees and Swale. Information is lacking mainly because the district is so thickly covered with Glacial and Recent deposits that, except at the extreme ends of this Tees-Swale stretch, rock is nowhere exposed. All the way from the Darlington country down to Catterick and beyond, for sheer lack of evidence the Trias—Magnesian Limestone boundary was assumed to be of normal, unfaulted character, but all was so uncertain that any borings in the area were likely to prove invaluable. How invaluable will be gathered from the following account of two recent bores, which throws an entirely new light, not only on the geological structure but also on the stratigraphy of this obscure region.