Three groups of moorsh formations distinguished on the basis of their ash content were divided into classes according to the state of progress of their secondary transformations. The basis for the division were numerical values of their water-holding capacity index W₁. It was found that with the increase of ash content, the threshold value of the W¹ index that distinguishes peats without any signs of secondary transformations from the peat formations secondary transformed, i.e., moorshes, also increases. In the moorshes with the lowest ash content (<25%) and weakly silted moorshes (26-50%), the threshold value of the W₁ index is 0.35, whereas in the strongly silted moorshes with the mineral part content from 51 to 80%, it is 0.40. Five classes were defined in each of the group of moorshes – the first of them includes moorshes at the initial stage of secondary transformations, and the fifth one totally degraded formations.