The aim of the paper is to analyse a specific case of transformation of communicative memories into cultural memories under the influence of state institutions in Teschen Silesia [Cieszyn Silesia; Těšín Silesia]. In 1920, the multi-ethnic territory of the former Duchy of Teschen was divided between Poland and Czechoslovakia. The author focuses on the state of local social memories after one hundred years since the division and raises the question, what kind of social memories are supported by the Polish and Czech local administrations and what kind of social memories are ignored or suppressed by them. Both sides of the Polish-Czech state border have at least one common feature today, namely the colonization of the past by the national states present here, in that the history of the Polish part of Teschen Silesia is commonly seen here as the history of Poland, and the history of the Czech part of this area as the history of the Czech Republic. It is an unconscious – and thus unreflective – conceptual structure that affects local national identities. The author calls this cognitive structure “demarcation logic”.