Geologists agree that it is ash but are divided on the origin of this layer.

The area under dispute is the Ghaggar Valley, Daghshai-Kasauli and the Shivalik hill sections. Contention swirl around particles called zircons, which vary from 0.05 to 0.2 millimetres.

Recently Panjab University geologists Dr A.D.Ahluwalia and his student Ritu Raj presented their research papers on the matter at a seminar organised by the Wadia Institute of Himalayan Geology.

The issue was first raised in 1984 when Prof S.K. Tandon and R. Kumar of Delhi University suggested that volcanic material originated from Dakhte-Nawar in Afghanistan, about 1000 kms from the site and was blown by the winds to various places including the Upper Shivaliks. However, three years ago, a senior faculty member rebutted the theory.

Writing on the discovery of tuffaceous mudstone in the Pinjore formations of Punjab sub-Himalaya, Tandon and Kumar predicted that tuffaceous mudstone would also be found in the Ghaggar valley, a little upstream of the Nada bridge.

While the volcanic nature of mudstones in the Jammu sector was relatively accepted, a doubt was raised about the origin of the tuffaceous mudstone at Pinjore by Prof L.N. Gupta of Panjab University in a keynote address at the Centre for Advanced Geology at the Geology National Seminar and Workshop on Neogene and Quaternary at Chandigarh in 1995. Prof Gupta managed to convince a significant number of people that the ash was not volcanic and that the origin of the material in the hills was certainly not Dakhte-Nawar in Afghanistan.

The scientific community was surprised. To quote a fellow scientist, Gupta had made "an ash of the ash horizon" ... till recently, when Ahluwalia and his student reiterated the thesis that the mudtsone is indeed volcanic.

The controversy largely revolves around minute zircon particles which are abundant in the mountain range. The nature of formation of zircons is also contentious. Gupta states that the glass shards (an indicator of volcanic origin) are not present in the area. Radiometric analysis points to a thermal event which has perhaps nothing to do with volcanic events, he says.On the other hand, Ahluwalia states that the presence of glass shards, which were dismissed by Gupta, is now reconfirmed, although their frequency is low. "The presence of full crystal zircons and their age positively point to a volcanic origin," he says.

Ahluwalia also dismisses the argument that it is difficult for the ash to have flown from Dakhte-Nawar and points to the wide dispersal of ash from the recent Mount St Helena eruption which was carried as far as 1500 kilometre away.

But the controversy is far from over yet. After all, a 2.4 million-year-old question is not something to be settled in a day!