Dear Kenneth,

In fact conditions necessary for preservation of those structures don’t have to be very specific. If raindrop imprints are commonly preserved in the geological record [in fact they are quite common in continental deposits around the world], why hailstone imprints should not be preserved. In the Precambrian, the frequency and preservation potential of hailstone imprints are expected to be higher than today. It is because the land was devoid of vegetation, making areas potentially affected by hailstorms markedly larger. Additionally, bare ground obviously produced larger dustiness of the atmosphere, and high temperature amplitude, what should promote the hailstone origin (nuclei for hailstone formation and vigorous weather respectively).

The so far investigations on the Precambrian atmosphere were focused on its potential thickness, density and chemical composition. The aspects of the dynamics of paleoatmosphere were simply overlooked, although its understanding can potentially exclude or promote some of the previously made assumption concerning the physical and chemical properties of the deep Archean atmosphere. It would be interesting for modeling purposes, how the different atmospheric composition, thickness and density influenced the dynamics of the paleoatmosphere and potential formation of cumulonimbus clouds in which hailstone originate.

Such a studies encompassing the problem of the dynamics of Precambrian atmosphere, have never been undertaken for obvious reasons – the hailstone imprints have never been reported form the geological record and they seem to be one of the sole feature that can be linked with the paleoatmostphere dynamics. And this is exactly what hailstone imprints can tell us.

Concluding, the hailstone imprints should be more common than we think. Why they have never been discovered – simply if you don’t know of how something should look like, how can you discover it??? – I hope that in near future there will be numbers of such discoveries…we will see.

Zbyszek

Dear Zbyszek,

This is an interesting topic, which reminds me of the phenomenon I found in Precambrian strata of the North China a few years ago. There are lots of pits in a dolomite bed, these pits are filled with dark mudstone(just like the pictures show). I asked a couple of colleagues and was told of "hailstone impression". And then a new problem arises up, what is the climate like if the hail happened with carbonate which seems to represent a warm climate. Additionally, what kind of structure is it if there were not hailstone impression?

It is found in the Tieling Formation of the North China, whose age is Mesoproterozoic about 1437Ma. The Tieling Formation is composed of carbonate with stromatolites and chert and a little shale, and rich in organic carbon with some oil indication and bitumen.

Dear Shouxian,

I think these are diagenetic structures, not of mechanical [hailstone] origin. If the pits or pans would be created before the dark mudstone, than the dark mudstone lamins should fill pits horizontally. When you look closer it is evident that the dark mudstone lamins lay parallel to the undulating surface of pits. This observation is also applicable to the next, overlying layers which also follow the undulating surface of the pits. Thus, the whole rock complex were immersed or squeezed giving different thickness of the dark mudstone. Maybe is the similar process as the genesis of the nodular limestones but less expressed.

Hello Zysbek, Prof. Towe and other colleagues,

I just want to put in common the link I already sent to you last month. This is the Journal of the Washington Academy of Sciences, where it is cited a report by A. J. Collier on fossil hailstone imprints in Montana, back in 1928 (no figurations, but it seemks that the author showed pictures of the imprints to the public):

http://archive.org/stream/journalofwashin181928wash/journalofwashin181928wash_djvu.txt

Best,

Jose Antonio

P. S.: The paper - that I mentioned to you - on Permian microichnofossils produced on pond shores after the rain, showing fossil raindrop imprints, can be downloaded in ResearchGate at:

https://www.researchgate.net/publication/264949190_Les_flores_du_Permien_basal_et_la_paloichnologie_de_la_fosse_de_Fombuena_%28province_de_Zaragoza_Espagne%29._5_Paloichnologie?fulltextDialog=true&ev=prf_pub_xaddfulltext

Article Les flores du Permien basal et la paléoichnologie de la foss...

Jose and Kenneth, 

Thank you for those information. I was simply unaware of them. If you will have any other, just let me know.

Best