This supply, so long as the marisma is dry, they are able to obtain from those raised ridges of calcareous debris (already described, and known locally as _vetas_) which here and there outcrop from the alluvial wastes. But when winter rains and floods have submerged the whole region and thus deprived the fowl of that local resource, they are forced to rely upon the sand-dunes aforesaid and to substitute pure sea-sand for their former specific of calcareous grit or disintegrated shells. To the sand-dunes, therefore, in the cold bright mornings between October and February, the skeins of greylag geese may be seen directing their course in successive files, in order, as the Spanish put it, "to sand themselves" (_arenárse_). A notable fact (and one favourable to the fowler) is that, though these dunes extend for miles, yet the geese select certain limited areas--or, to be precise, the summits of two particular hills--for alighting, and this despite their being regularly shot thereat, year after year. With the first sign of dawn the earlier arrivals will be heard approaching; but the bulk of the geese come in about sun-up and onwards till 9 A.M. Geese arriving high (having come presumably from a distance) will sometimes, after a preliminary wheel, suddenly collapse in mid-air, diving and shooting earthwards in a score of curving lines--as teal do, or tumbler-pigeons; but with these heavy fowl the manoeuvre is executed with surprising grace and command of wing. Their numbers vary on different mornings without any apparent cause; but it may be laid down as a general rule that more will come on clear bright mornings than when the dawn is overcast, while rain proves (as in all wildfowling) an upsetting factor. Sometimes, even on favourable mornings, no geese appear. Occasionally, in small numbers, they may visit the sand in afternoon. To exploit the advantage afforded by this habit of the geese, it is necessary that the fowler be concealed before dawn in a hole dug for the purpose in the sand--care being taken to utilise any natural concealment, such as a depression flanked by a steep sand-revetment; so that, at least from one quarter, the geese may perceive no danger till right over the gun. The hole (or holes, but _one_ is best) must be dug at least twelve hours before, or the newly turned sand will show up dark. Were it not for the risk of wind filling them up with driving sand (a matter of an hour or two), the holes might well be prepared two or even three days beforehand. The excavated material is piled up around the periphery and flattened down smooth, thus forming a raised rampart which screens the suspicious darkness of the interior. Needless to say, the fewer human footprints around the spot, the better.