The large no ordinary mud mounds were discovered in summer 1993 in the Lo­wer Ordovician Volkhov stage limestones of the Putilovo quarry. The mud mounds have sparitic flanks, micritic crusts and a clay in their cores. We suggest to call them the "Hekker-type mud mounds". There is a possibility that little sinsedimentary humps discovered under the Ivangorod fortress and in the Kingisepp quarry are embryonal forms of the Hekker-type mud mounds. The humps are very similar to the sedimentary
folds" that were described in Sweden by M. Lindslrom.