The accumulation of vertebrate mostly mammal skeleton fragments in the "Bone Valley" at Hajnacka — a type locality of the European Neogene Mammal time-scale, zone MN 16 and/or subzone MN 16a came into existence in a lake with water influx and outlet. After cessation of the phreatomagmatic eruptions responsible for the maar creation, the maar was filled by the finely laminated sediments. The domatic rise of the Cerova vrchovina Upland motivated the erosional destruction of any relatively soft relief protuberance. The ring of the maar was partly destroyed and the sedimentary maar fill was swept out. Later on the emptied maar was filled by water. In the lake originated in this way sandy sediments and tuff were deposited together with the bones of mammals, killed by postvolcanic gas emanations, or tephra fall, when the animals drank the water of the lake. The age of the subzone MN 16a is 2.8–3.3 Ma BP (Fejfar & Heinrich 1987). The subzone corresponds to the middle part of the chron C2An. The maar originated earlier in the early period of the same chron, because the tuff has a normal magnetic polarity. It could not be generated before 3.55 Ma BP — that is the numerical age of the chron C2An/C2Ar, because the chron C2Ar lasting 0.6 Ma (3.55–4.15 Ma BP) was a period of the reverse polarity of the Earth’s magnetic field. From the comparison of the age of the bone accumulation in the Hajnacka maar and the basalt lava flows of the Cerova Basalt Fm. it follows that effusive activity acted before both phases of the maar creation and its filling as well as after it.