This study seeks to describe ‘baksteenkalk’, an erratic silicified bioclastic carbonate of the Upper Sandbian from the eastern part of the Netherlands. To date, baksteenkalk has received little attention among palaeontologists. This is to be regretted on two grounds. First, baksteenkalk contains a varied fossil flora and fauna comprising many species, several of which are not or only rarely found in coeval rocks. Second, owing to a complicated silicification process, fossils, in particular algae, have preserved exceptional anatomical details. The primary aim of this study is to arouse the interest of professional, in particular, Estonian, palaeontologists in baksteenkalk. Based on lithology and fossil assemblage (most conspicuously, with regard to the algal flora), two basic types of baksteenkalk are distinguished. A list of species, differentiated for both types, is provided. It is argued that baksteenkalk reflects the ecology of a shallow, subtropical, epicontinental sea. The distribution of erratics, facies and fossil content point to an origin within the North Estonian Confacies Belt, probably west of Estonia. Baksteenkalk survived as an erratic because it was already silicified at its place of origin. A potential source of silica may have been Upper Ordovician bentonite layers. The causes and mechanisms of the silicifi­cation process which gave shape to baksteenkalk are not yet understood, however. The palaeontology of baksteenkalk is compared with that of two other erratic Sandbian silicified carbonates of Baltic origin: German Backsteinkalk and ‘Lavender-blue Hornstein’.