I have already mentioned here and there the fact that the famous "stone balls" (kamene kugle) that, according to Mr. Osmanagic, exist nowhere else than in Bosnia, in Mexico and in Costa Rica, are in fact a phenomenon much more frequent than is usually known, as they can be found on all the continents, including Europe. I recently discovered [1] that they are also present in France, in Saint-André de Rosans in the Hautes-Alpes.

The visitor coming to Saint-André de Rosans cannot miss, on the village place where are the Mairie, the Monument to the deads of World War I, and the little inn (where Monique (fr) will serve him a delicious meal if he takes the trouble to warn her the day before...), this strange object:

A walk around the village will show him some others, scattered among the gardens, and if he goes a few hundreds meters on the road 425 toward the Col de Palluel, he will be able to see tens of balls in a large outcrop alon the road:

A little bit higher on this road, the visitor will find the hill Serre d’Autruy:

There, a small track leaving the road on the right will take him on a sandstone plateau, where he will be able to discover a few hundreds balls, either on the plateau itself:

or protruding from the escarpment face:

What is the origin of these balls, that the local inhabitants sometimes wrongly call "dinosaurs eggs"?

Before explaining the geologist’s point of view, let’s begin with a few ground notes. The balls diameter is always quite important, at least a few tens of centimeters, and can be, for the biggest I could see, almost 2 meters. Their shape, always rounded, can vary from the ovoid to the perfect sphere. These balls are clearly incased in the sandstone layer, which means that a human origin is totally excluded; one can see below the contact between a ball and the encasing rock:

and some instances of balls solidly encased in the massive sandstone layer:

These balls, even if they seem quite sensitive to frost weathering, are clearly more resistant than the encasing rock, which explains how they can be little by little bared and extracted from the layer by the weathering, leaving behind themselves a nice imprint:

and they sometimes happen to roll quite a long distance in the marly gully under the sandstone escarpment:

As long as the balls are more or less protected from the weathering in their sandstone matrix, their surface is quite even:

But as soon as they are exposed to the region aggressive climate (the temperature is very frequently much negative at 800 m during winter in this part of the Hautes-Alpes), and because of the almost total lack of soil and vegetation, their surface is swiftly degraded, and the entire ball will progressively go to fragments with the frost effects:

One can see here for instance the contrast between the lower part of the ball, that was encased until recently, and the upper part, long exposed to weathering:

Finally, let’s note that they frequently offer a characteristic structure, with more or less concentric layers:

and that their composition seems to be very similar to that of the encasing sandstone, from which they differ only by the color, often more somber, and by the hardness.

The sandstone bank containing these balls in Saint-André de Rosans has formed more than 110 million years ago, during the Secondary era, more precisely during the Lower Cretaceous, and more precisely still during the Upper Aptian. At this time, long before the uplifting of the Alps, all the region (the "Baronnies") was on the slopes of a submarine "trough", called by the geologists "fosse vocontienne" or "Vocontian Trough": see on this subject the excellent work by Gérard Friès and Olivier Parize, published in 2003, "Anatomy of ancient passive margin slope systems: Aptian gravity-driven deposition on the Vocontian palaeomargin, western Alps, south-east France" [2], whence come most of the informations in this text about the Rosans sandstones. This submarine slope, extending between the shallow seas on the continental shelf of the time (present Vercors and Vivarais) and the deep oceanic basins of the former Tethys, received an intense sedimentation, mostly marls (the Aptian and Albian "Blue Marls", resulting from the deposit of mostly fine muds and silts) of which an instance can be seen here:

But it happened regularly on this slope some more "turbulent" moments, submarine slumps and avalanches that could locally (and particularly in the valleys and canyons that dissected the slope) bring massive deposits of more coarse sediment. During one of these moment a sandy avalanche has filled the channels incised into the marly slope for nearly 50 km downslope, and is, according to G. Friès and O. Parize, responsible for the formation of the Saint-André de Rosans sandstones.

Again according to these authors, the sandstones resulting from the lithification of this sandy avalanche are constituted mostly by quartz grains, with a non trivial part of glauconite (responsible for the often greenish color of the sandstones) and sliceous and carbonate debris (shells). An important feature is their massivity, and the almost total lack of internal sedimentary structures [3].

The sandstone balls are mentioned in the above work as "diagenetic doggers", or diagenetic concretions: it means that their formation took place after the deposition of the sands, but before the complete lithification, that is during the long period when, under the growing pressure and heat due to their burying, the sands underwent the cementation and compaction that changed them into sandstone. During this period there are numerous phenomena of matter redeposition and crystallization inside the lithifying sediment, and the crystallization is aided by the previous existence of a "germ", calcareous debris or shell for instance, that will "attract" the crystallization of the carbonates. From this "germ", the growth of the ball by successive crystallization will develop equally in every direction as (see above about the lack of sedimentary structures) the sediment is quite homogenous, without any important heterogeneity that could hinder this regular growth. The way of this growth can explain the balls structure in concentric layers.

This same massive sandstone bank of the Upper Aptian, although quite dismantled by erosion since the uplifting of the region, can be found in various parts of the Baronnies, where similar sandstone balls have been mentioned. Their existence is attested in Bourdeaux, Arnayon, La Charce, Bevons, Châteaneuf-de-Bordette, Condorcet... I myself have seen some in Saint-Ferréol-Trente-Pas, where the sandstone layer forms an outcrop above the place called Le Monestier, and regularly "releases" balls of various size that the inhabitants use as ornamentation:

But nowhere these balls are as beautiful and numerous as in Saint-André de Rosans. If you dream of a 110 million years travel back in time, and have the ability over a fossil sandy avalanche and a surrealist landscape, then you are the one for a little walk on the Serre d’Autruy!