The article discusses the origin of wedge-shaped structures observed on southern lowland of the Gulf of Finland near the village of Nizino in 2000-2005 and 2014. Obtained results confirm the seismic origin of the wedges. Morphological characteristics, nature and consistency of the filling, deformation of the enclosing strata show the instant disclosure, a quick one-act filling cavities with material from the sides and roof, the subsequent momentary closure with compression in connection with the directed lateral dynamic effects. In addition, we analyzed the distribution of various deformation structures in Paleozoic rocks and postglacial sediments within this region and arrived to the conclusion about their structural control and paleoseismic origin.