Several very large synsedimentary slide units exist in the well-exposed Mesozoic fore-arc sequence of Alexander Island, Antarctica. The largest single exposure, which is at least 440 m thick and more than 21 by 6 km in area, forms part of a unit that has a volume of as much as 300 km3. These units are as large as slide deposits noted by remote sensing on modern continental margins. The apparent absence of large, ancient slide deposits is therefore purely a function of exposure. The enormous size of these Antarctic examples, in which sheets of partially lithified sediment, up to 1 km long, have been transported with little or no internal deformation or tilting, emphasizes the care needed in determining that even very large outcrops are not allochthonous.