A unique World War II battlefield artifact fused to a beach deposit

Coasts are areas where specific sedimentary formations referred to as beachrocks are formed (Vousdoukas et al., 2007). They consist of different beach sediments, mainly cemented by calcium carbonate. Various artifacts of human origin, such as plastics, slags, fragments of bricks, metal objects and glass, have been found in beachrocks (Fernandino et al., 2020; Gestoso et al., 2019; Santos et al., 2022; Turner et al., 2019; Wang & Hou, 2023). Emery et al. (1954) have reported the occurrence of World War II (WWII) relics such as brass cartridges and shells cemented in the beach sediments of Eniwetok Atoll where military operations took place. Many shells and aerial bombs from WWII were found on Polish beaches near Kołobrzeg, Mielno, Darłówko, Jastarnia and Hel after storms in the 1950s, 1960s and 1970s (Szarejko & Namieśnik, 2009).

Poland is one of the countries that have been most heavily affected by remnants of munitions from WWII, with over 200,000 items located and destroyed each year from 1945 to 1985 (Molski & Pająk, 1985). Multiple accidental findings of ammunition washed out from the Baltic Sea shelf on Polish beaches, including those in Kołobrzeg, were reported over a time span of the 1950s through 1970s (Szarejko & Namieśnik, 2009). However, no specimen composed of a bullet fragment and beach debris has ever been reported.

The military artifact described in this paper is most likely a remnant of the fierce fighting that broke out between Polish-Soviet armed forces and German troops that defended the strategic fortified town of Kołobrzeg from March 4 to 18, 1945 (Baxter, 2025). The Battle of Kolberg was a key part of the East Pomeranian Offensive, aimed at clearing German forces from Pomerania and West Prussia.

Both sides used all types of conventional weapons on land and sea, inflicting heavy casualties running into thousands. According to rough estimates, the Polish-Soviet military units used over 100 guns per 1 km of the front line, and during 10 days of fierce street fighting, about 43 tons of ammunition (approximately 31,500 pieces of bullets) were fired per 1 km² (Zdziech, 2009).

As a consequence, there is a great amount of munitions sunk in the Baltic Sea. Łabuz (2021) reported that in 2007, storm surges of water had washed out ammunition and shell cases from the seabed on the Kołobrzeg beach. Military waste is also washed as a result of shore erosion that occurs on the Baltic Sea coast (Łabuz, 2021). There was only one bigger storm surge (sea level rise ≥70 cm from the reference gauge) in Kołobrzeg in 1996, but nine storm surges occurred in 1995 (Wolski & Wiśniewski, 2012). However, it is not possible to link washing up the studied specimen with a specific event.

The aim of this study is to present an explanation for the formation of the unique specimen based on the results of our petrographic and chemical analyses, as well as to discuss its components in the context of local geology and WWII history.

The compact specimen of 4.5 cm × 3.5 cm in size is tripartite. It consists of: (i) a ferruginous micritic limestone pebble, semi-rounded, light brown attached to (ii) a brown to dark brown/black vari-grained ferruginous quartz-limestone sandstone with single limestone pieces up to 10 mm across, and pierced by (iii) a worn brassy ∼13-caliber bullet. The hole in the bullet fragment suggests that it is part of the bullet case (Fig. 2). All three parts of the specimen are shown in Fig. 3. The bullet fragment lacks any manufacturer’s marking, and it is difficult to attribute the bullet to a specific gun. However, based on the size and possible caliber, we speculate that the brass fragment belongs to the bullet used in 13 mm aircraft machine guns, specifically the MG-131 (Rheinmetall-Borsig, Germany), which were used in the Battle of Kołobrzeg.

Figure 2.

Major components of a bullet.

The limestone pebble is composed of calcite with subordinate silty quartz grains and dispersed iron oxyhydroxides and clay minerals. The most interesting is the central part of this specimen, which is made of detrital quartz and limestone grains bound with iron oxyhydroxides, clay minerals, and subordinate micritic calcite (Figs. 3C,D; 4). Conglomeratic quartz-limestone sandstone is composed of fine-grained quartz averaging ∼0.2 mm across with limestone and subordinate sandstone pebbles up to 1 cm in size, cemented with iron (hydro)oxides. Quartz grains are covered with black efflorescence. The gun bullet is surrounded by a black halo of ∼0.5 mm in size, composed of quartz grains cemented with manganese-iron oxides in some places, forming botryoidal micro-occurrences (Figs. 3A,B). This sintered boundary zone is devoid of clay minerals and micritic calcite. This may suggest heating to a temperature below the melting point of basic detritic specimen ingredients, i.e., quartz grains and limestone particles. However, the temperature was high enough to decompose micritic calcite, and moreover, to recrystallize Fe/Mn oxyhydroxides into their oxide equivalents. In addition, the boundary between a limestone pebble and a conglomeratic sandstone reveals the presence of yellow coloration, which may also indicate heating (Fig. 4).

Figure 3.

(A) Macrophotograph of the worn machine gun bullet embedded in a conglomeratic Fe/Mn-rich quartz-limestone sandstone bound to a semi-rounded micritic limestone pebble (+HCl). Specimen microphotographs: (B) upper part of the worn bullet encircled by a sintered Fe/Mn-rich halo, (C) a fragment of the bullet-sandstone boundary with a distinct Fe/Mn-rich streak, (D) a ferruginous sandstone (center) in direct contact with a limestone pebble (left) and a bullet fragment (right).

Figure 4.

(A) Macrophotograph of a ferruginous limestone pebble in direct contact with a conglomeratic sandstone. (B) Microphotograph showing a more detailed microtextural relationship between the conglomeratic sandstone and the limestone pebble; note that the yellow ferruginous streak separates these two petrographic components, which may also point to heating.

Brass used in ammunition manufacturing consists of copper 65%–74% and zinc 26%–35% with a trace amount of additives, including Ni, Fe and Pb (Wallace, 2018). However, a different brass composition is also possible with a share of copper from 50% (beta brass in castable) to 95% (gilding metal in ammunition) and zinc from 5% (in different applications, such as coins, cast parts, and ammunition) to 50% (beta brass in castable) (Benvenuto, 2016). The results of chemical analyses support the data obtained from petrographic examinations. Concentrations of major and trace elements within three discriminated parts of the specimen are presented in Table 1. Ferruginous limestone shows the highest concentrations of Ca (36.3 wt.%), Si (2.41 wt.%), Fe (1.24 wt.%) and K (0.364 wt.%). Conglomeratic sandstone contains 14.5 wt.% Si, 14.6 wt.% Ca, 8.37 wt.% Fe, 0.420 wt.% K, showing additionally an enrichment in sulfur (0.804 wt.%) and titanium (0.223 wt.%). Results derived from the XRF analysis of the upper brass part of the examined specimen are probably underestimated because the sample instrument window partly covers an empty space (a hole in the worn bullet case). Because the bullet fragment was exposed to atmospheric conditions, metal contents could have decreased over time due to weathering. There is also a possibility that the lower-cost alloy was used for the production of the bullet cartridge. A shortage of metals during WWII and a need for reduction of production costs may have caused brass in bullet cartridges to be replaced, for example, by steel covered with copper (Petrie et al., 2008).

As it has already been mentioned, military artifacts from WWII are often found on the Baltic coast, including beaches in Kołobrzeg (Kobalczyk, 2011). This is the reason why tides and currents brought this specimen to the surface.

The process of the examined specimen formation is unclear and difficult to explain. The temperature of a fired bullet casing for a few seconds after firing is about 60°C (Gashi et al., 2010), whereas the melting temperature of brass is about 920°C (Davies, 1993). The examined specimen shows marks of thermal alteration of a brass fragment (Fig. 3). Thus, an external source of heat should be considered, such as a battlefield fire with high burning temperatures (Kim, 2000).

The specimen examined in this study shows features of a rock-like material composed of three distinct parts, namely, a ferruginous micritic light brown limestone pebble rigidly attached to a dark brown ferruginous conglomeratic quartz-limestone sandstone and a worn brassy ∼13-caliber bullet. Rocks that were fused with a brass bullet fragment represent gravel typical of beach deposits in this location. This agglomeration of constituents and their characteristic microtextures would not be possible without the impact of a high-temperature process that led to partial melting of the brass alloy and pyrometamorphism of micritic calcite, and recrystallization of Fe/Mn oxyhydroxides into Fe/Mn oxides. Based on the results of this study, it is not possible to conclude whether the whole rocky material had been partly compacted before the bullet impact that changed its matrix, or the originally loosely-bound gravel-sand rock became hardened as a result of these thermally-induced diagenetic processes. This preliminary study may serve as an incentive to search for similar hybrid conglomerates in other areas worldwide.