2.2 Sima de las Palomas del Cabezo Gordo

Sima de las Palomas del Cabezo Gordo ("Dove Hole on the Big Hill") lies in utterly different surroundings (Plate 5, Plate 6). Twin mouths of this natural karstic shaft lie at 120m asl on the southern slope of a hill 312m high that sticks out, like the proverbial sore thumb, in the middle of Murcia's coastal plain. It overlooks a large lagoon 6km away (Mar Menor - 'Lesser Sea') of the Mediterranean Sea, and is surrounded by irrigated lemon-tree plantations. It dominates the village of Dolores de Pacheco in Torre Pacheco municipality. Annual sunlight is about 3,000 hours a year. Mean July temperatures rise to 26ºC, and mean January ones rarely fall below 10ºC (Figure 2); almond trees bloom in late January. This is the hyper-arid Murcia-Almerian biogeographical province of thermophyllous (warmth-loving) scrub that includes some African taxa in its thermomediterranean flora. Pollen of this most unusual of all European floras was present even during the Upper Pleistocene at Perneras and Algarrobo Caves (Carrión et al. 1995; Munuera and Carrión 1991). Physical obstacles to sampling pollen at Sima de las Palomas have now been overcome and Dr José Carrión will begin sampling our 18m deep sequence in 1998.

Plate 5: Cazebo Gordo from the south. Sima de las Palomas lis on this side of the hill. (Photo M. J. Walker)
Plate 6: The main chamber below the deepest of the two entrance shafts of Sima de las Palomas before the scaffolding tower was built here. (Photo M. J. Walker)

Sima de las Palomas was emptied of most of its Pleistocene fill by iron miners 100 years ago who doubtless hoped a conspicuous vein of magnetite beside one mouth would give rich pickings, whereas it swiftly petered out. Perhaps they nevertheless persevered in their Herculean task, less out of perversity than to gain access to precious water that may formerly have lain in the now dry terminal shaft of the small cave system, before artesian wells - sunk early this century - lowered the water table around the foot of the hill. The miners went to the trouble of blasting out a horizontal mine level for 20m through solid rock to facilitate access to the main chamber below the twin entrance shafts (Plate 7, Plate 8); the tunnel's wall shows holes drilled out for explosives. Beneath the magnetite vein they built up a stone revetment, inside the main chamber, for a platform or landing between wooden ladders up which they carried the rubble they had so laboriously dug out. They left no written documents about their work.

Plate 7: Mine level entrance on the hillside. (Photo M. J. Walker)
Plate 8: Mine level. (Photo Paul Mann)

Cabezo Gordo is an isolated Triassic limestone and dolomite massif (of the Nevado-Filabrid Complex), in which Triassic and perhaps Neogene metamorphism produced marble, crystalline (recrystallized?) calcite, amorphous calcite, greyish-white micaceous quartzite, white quartz, transparent rock crystal, micaceous slate, micacite, greenish micaschists, magnetite, iron hydroxides, iron pyrites, and copper and manganese oxides (Colodrón et al. 1994); local metal ores mainly owe their origin to end-Tertiary vulcanism (Fernández Gutiérrez 1986 and refs.). East-west faults separate the hill's steep northern peak from its elongated southern limb at a saddle lying at 175m asl where quartz and micaceous slate outcrop in beds a few metres thick. No flint sources have been found anywhere, though mining and quarrying may have eliminated them or rubble heaps covered them: none are known to quarrymen at a vast century-old quarry (unprofitable magnetite mining ended around 1913; de Galvez-Canero 1913), nor to local naturalists keen to conserve the hill, which we too have scoured. Oddly, neither continental nor marine terraces line the hillside, though Quaternary marine terraces and continental formations abound both to its north and inland to its east.

Plate 9: Sima de las Palomas: right side of CG-1 mandible and fragmented maxilla. Scale in cm (Photo M. J. Walker)
Plate 10: Sima de las Palomas: left side of CG-1 mandible and fragmented maxilla. Scale in cm (Photo M. J. Walker)

Fused upper and lower Neanderthal jaws (Plate 9, Plate 10) were plucked out from high up in breccia (Figure 5) by a local conservationist, Juan Carlos Blanco Gago, abseiling down the main entrance shaft in 1991 (Gibert et al. 1994; Walker and Gibert in press). A vertical cliff overhangs the 18m high face of an inaccessible column of breccia - left behind by the miners against the rear wall of the shaft, 3m out of reach across the abyss from the outer lip of the shaft mouth. Nor could the breccia be reached from below, because rubble had run into the horizontal mine level at both ends.

Figure 5: Sima de las Palomas: horizontal plan
Figure 6: Sima de las Palomas: vertical elevation

The outer lip of the shaft mouth forms a low wall of frost-shattered blocks cemented together by calcium carbonate that also must have formerly covered the entire breccia fill in the mouth because traces of calcrete flowstone form a rim adhering to the overhanging precipice. Maybe when miners began using explosives (around 1875 in Murcia), shock waves caused collapse of the upper mouth's fill that they had perhaps left alone in its perched situation, slightly offset from a main chamber (Figure 6) they had emptied using the less high entrance (beside the magnetite vein) before driving the mine level. The breccia consists of cemented soil (lutite, silt and sand) with frost-shattered blocks and scree that fell into the shaft. The column is not quite vertical and seems to comprise three major units, each 5-6m thick, separated by discontinuous features (Figure 5).

Traces of breccia are stuck to the roof of a narrow passage. This descends from the main chamber to a terminal shaft that ends in impenetrable argillaceous sediment lying at a height above sea level similar to that where a dry gully begins descending the hillside beside the mine-level entrance. Maybe springs once welled up to feed the gully, most likely fed by hydrostatic pressure from a vast saucer-shaped aquifer in Triassic rocks that dip underneath the plain below. Such rising water might thus have reached the foot of the terminal shaft from below, even though the cave above was full of breccia. Modern annual rainfall of 175mm (min. 90, max. 350) (cf. Figure 2) makes it unlikely much surface water descended the shafts in recent Quaternary time (though it obviously did so in some past geological epochs because a clearly vadose passage i.e. formed by a underground stream now long-vanished, leads down from the main chamber to the terminal shaft).

The main chamber's floor contains some 2m of rubble and soil beneath the point where the mine level enters it. The miners doubtless dumped them there after digging them out from the terminal shaft (and connecting vadose passage) before they drove the mine level through the rock from the hillside to the main chamber. Under rubble and soil close to where the mine level enters the main chamber, there is a hint of a small natural passage heading outside (Figure 5, Figure 6); we're exploring this possibility because a low access to the main chamber during the Upper Pleistocene would carry significant implications. After heavy rain there are many signs that water running down the main shaft drains into this corner of the main chamber, and not, strangely, the opposite corner which lies nearer to the terminal shaft. Nevertheless, no signs exist of any temporary springs either in the gully or on the hillside beside it. This might corroborate a conjecture of sporadic "topping up" of an inaccessible, lower, karstic phreatic network in which the water level is no longer sustained by an aquifer because the cave is nowadays completely dry, as is the nearby gully. Indeed, a total absence of water near the site meant that wet-sieving had to take place a few kilometres away.


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Last updated: Wed Dec 23 1998