The picture stones are made of local Gotlandic limestone, a few of sandstone. The raw material was chosen according to its suitability and availability. The unique features of the limestone, like fossile inclusions, were sometimes deliberately integrated into the image design. In a few cases, it seems that a particular slab was chosen because it had the right surface structure for the intended image. On the other hand, the irregular surface of the limestone can make it difficult to interpret the imagery.

The geology of Gotland

Gotland’s geology is widely known among geologists and paleontologists for its easily accessible limestone outcrops with its wealth of fossile inclusions.

Geologically, Gotland is a plateau which is built up mainly of limestone, marl, and a small amount of sandstone. Such plateaus dominated by limestone are called carbonate platforms. Gotland is dated geologically into the Silurian (443 to 419 million years ago). During that period, it belonged to the continent of Baltica which was the precursor of Europe, and was positioned on the equator. Many reefs were built by calcareous algae, calcareous sponges called stromatoporoids, corals, and small moss-like animals called bryozoans. These reefs were inhabited by snails, clams, slater-like animals called trilobites, clam-like animals called brachiopods, and flower-like animals called crinoids. Early ancestors of squids, large sea scorpions, and early fishes swam in the water on and above the reefs. The fossils of these organisms can be found everywhere on Gotland, and they build up the rocks there.

The remains of the organisms, which have skeletons of the stable version of lime called calcite, are still preserved. This makes the measurement of the geochemical fingerprint possible which is the isotope signature of the Silurian seawater. Both the carbon and the oxygen isotope signatures show fluctuations which are indicators of a climate and an environment changing frequently. This pattern allows for dating the rock layers on Gotland relatively.

On Gotland the layers of the Silurian rocks are exposed in a sequence which starts on the north-western coast with the Lower Visby Formation and continues towards the south until the Hamra-Sundre Formation. Gotland is dominated by three reef areas, the Tofta Reef following the north-western coast, the Hemse Reef towards the east, and the Hoburgen Reef with atolls at the southern end. Around and between the reefs, thick layers of reef debris containing the organisms from the reef are deposited. Close to the single reefs, especially in the north western part and in the middle part of Gotland, thick beds of limestone are exposed containing less or no reef debris. Between these three reef areas, alternations are built up by thinly layered limestone and marl. They indicate deeper water conditions during some periods of the Silurian.

Gotland is the most important archive for the Silurian. The preservation conditions of the fossils, especially the calcitic ones like brachiopods, are perfect, and the rocks did not get any deformations. These lucky conditions on Gotland are very rare in other areas on Earth, and make Gotland so important for geologists working on the Silurian period.

Changing preferences

The picture stones on Gotland are made mainly of thickly bedded limestone. Among the more than 570 known picture stones, only 23 ones are made of sandstone from the Burgsvik area (see list). Small and large slabs can easily be extracted from the thick beds of limestone which are perfect for the production of picture stones.

The thickly bedded limestone which was used for the picture stones is mainly reef debris limestone. This kind of limestone contains remains of reef-building organisms like calcareous algae, calcareous sponges, corals, and the moss-like bryozoans. The reef debris build up thick layers around the reefs. For the A stones and the (rare) D stones, fine reef debris limestone with reef debris pieces up to 1 cm was preferred. Coarse reef debris limestone containing reef debris pieces larger than 1 cm was preferred for producing the C stones and the special cist stones. The E stones show no clear preference for a special kind of thickly bedded limestone.

A special observation is that the B stones are mainly produced of crinoidal limestone. This thickly bedded limestone contains skeletal parts with a size up to 5 mm of flower-like sea animals which are called crinoids or “sea lilies”. Their skeleton pieces are single crystals which cause a sparkling effect of this limestone in sunlight. The preference of this limestone follows probably a fashion trend which appeared during the time when the B stones were produced. Crinoidal limestone is also delicate. The production of large picture stone out of this limestone is difficult, and maybe this is the reason why the B stones are relatively small.

Picture stones and carbon isotope signature

The Silurian rocks on Gotland have a stable carbon isotope signature (δ¹³C). This signature show some changes from one geological unit to another. For example, the rocks of the geological unit Slite Group have negative carbon isotope values, and the rocks of the geological units Eke Formation and Burgsvik Formation have high, positive carbon isotope values of more than 9 ‰ (per mille).

By comparing the carbon isotope values of rock pieces collected in the field and picture stones, the raw material of any picture stones can be traced back to its geological unit by its carbon isotope values.

Local raw material

The comparisons brought an interesting result: The crafters preferred local materials for the picture stones on Gotland. They went directly to the Silurian reefs for collecting slabs of thickly bedded limestone. These pieces of limestone could be taken directly on the surface in the landscape and especially on the coast. It was not necessary to create a quarry. Only a few picture stones were transported over some distance. Here, the carbon isotope signature of the raw material is not comparable to the carbon isotope signature of the rock which is exposed at the find place, but it fits to the rocks of a geological unit which is exposed far away from the picture stone’s place. The stone was transported 10 km, or even up to 40 km. Examples are the A stone GP 159 Halla kyrka, the B stone GP 93 Gammelgarn Högstens, the C stone GP 209 Klinte Hunninge I, the C stone GP 79 Etelhem järnvägen, the D stone GP 280 När Smiss I, the E stone GP 208 Hogrän kyrka I, the E stone GP 356 Sjonhem kyrka II, the eroded C, D, or E stone GP 154 Halla Broa 21, and the cist stone GP 103 Garda Smiss II. It is hard to tell the reason why a picture stone was transported over a large distance. Maybe the person, for whom the picture stone was dedicated, came from the raw material origin area, or the crafter found the raw material there more suitable than the raw material exposed on the standing place.

How to understand the geological results

These geological results and observations of the picture stones’ raw materials should not be misunderstood: The carbon isotope signature allows only a relative geological date. This means that the limestone can be assigned only to its geological unit where it was originally exposed. All estimations of transport ways are based on the distance between the find place and the closest fitting raw material source. It is not possible to determine the exact spot where one specific slab was extracted within this geological unit.

Information in the online edition

In the online edition, you will find information like “coarse” and “fine”, and percentages for the probability of the origin are given.

The distinction between “coarse” and “fine” for the thickly bedded limestone is based on the technological limestone texture which was more important for the crafters than the geological texture of the limestone, called facies. Choosing the suitable raw material for a picture stone was dependent on the texture because of the production process. For getting a visually attractive raw material, the fossil content was also important.

The percentages given in connection with the raw material’s origin are probabilities which were estimated in a complex comparison, on the one hand with the carbon isotope values of the field samples and of the picture stones, and on the other hand with the carbon isotope values of the picture stones and of the rocks being exposed on the find place. Some geological units have a similar carbon isotope signature which decreases the probability. The more similar these signatures are, the lower are the probabilities. For example, the Slite Group has the highest probability percentage because only this geological unit shows negative values. The geological units Högklint Formation, Tofta Formation, Hangvar Formation and Hamra-Sundre Formation show a low probability percentage, because they have a very similar carbon isotope signature.

One hundred and seven picture stones have been sampled and their raw material was also determined in a technological aspect. For nine further picture stones, only the raw material has been determined yet. The raw material of the other picture stones made of limestone can be determined in a comparison of the photos with the 3D scans.

PH

List of picture stones made of sandstone:

A stones: GP 115 Grötlingbo Barshaldershed Uddvide 1:20, GP 116 Grötlingbo Uddvide.

B stones: GP 232 Levide Backänget Pejnarve 1:59.

C stones: GP 13 Alva Bopparve, GP 203 Hemse Annexhemmanet II, GP 204 Hemse Annexhemmanet III, GP 480 Stånga kyrka 1.

E stones: GP 22 Ardre kyrka III, GP 23 Ardre kyrka IV, GP 20 Ardre kyrka VII, GP 113 Grötlingbo kyrka, GP 123 Grötlingbo Roes I, GP 279 När Mickelgårds.

Eroded C, D, or E stones: GP 55 Burs Prästgården, GP 80 Fide Österby, GP 117 Grötlingbo Råirhage 1:1.

Kerb stones: GP 236 Linde Duckarve.

Cist stones: GP 15 Ardre kyrka I, GP 16 Ardre kyrka II, GP 17 Ardre kyrka V, GP 18 Ardre kyrka VI, GP 69 Eke Smiss, GP 201 Hemse Annexhemmanet I.