Markings in mud at Ruafa, North Sinai In general, photography in the tropics offers no great difficulties and demands no special precautions, with the single exception that all sensitized materials must be kept in sealed moisture-proof tins, and must be kept from extremes of heat.

SOME CURIOUS MARKINGS IN MUD AT RUAFA, NORTH SINAI

T

HE Wadi el

T. Sutton Bowman

Arish drains between one-third and one-quarter of the Peninsula of Sinai into the Mediterranean. This wadi is dry at ordinary seasons, but conveys seils (flood streams) during the infrequent and heavy rains which visit the region. At Ruafa, a waterhole some 35 miles south-east of the town of El Arish, the wadi passes through a narrow gorge in an outcrop of chalk with flints (of either Lower Eocene or Senonian age), and here the Turks had started to build a dam during the late war.

In the spring of 1924 there were exceptionally heavy rains in Northern Sinai, and the wadi ran in seil for three days, attaining such a volume and violence that the railway, which crosses its mouth near the town of El Arish, was swept away. The writer visited Ruafa in August 1924, and found that a pond, which had been excavated in the east side of the wadi bed to serve as a reservoir in connection with the dam referred to above, had been filled to a considerable depth with the mud brought down by the seil. This mud had hardened and shrunk into blocks of an average width of about 60 centimetres. The cracks between the blocks were from 5 to 10 centimetres, and extended to a depth of about 50 centimetres, the mud being still plastic at that point. It is noteworthy that the blocks were all of an irregular form, and that none approaching the hexagonal shape were seen.

The fine yellow clay composing these blocks was highly calcareous, and was close textured (though porous) and homogeneous to within about I centimetre of the top, at which point the texture became more open, and the clay riddled with small sinuous, flattened, tube-like channels, the length of which lay horizontally rather than vertically. On treatment with hydrochloric acid and washing in water, the clay yielded a small quantity of finely divided vegetable matter. The uppermost 2 millimetres of the clay, forming the actual surface, was covered with small raised irregularities, not more than I millimetre in height, and small holes (like pinholes), the whole presenting a frothy appearance.

As shown in the accompanying photograph, which was taken vertically from a height of about 12 feet, the surface of every block was covered with a perfectly regular series of larger circular depressions, averaging 4 centimetres in diameter and I centimetre depth. These Markings in mud at Ruafa, North Sinai In general, photography in the tropics offers no great difficulties and demands no special precautions, with the single exception that all sensitized materials must be kept in sealed moisture-proof tins, and must be kept from extremes of heat.

SOME CURIOUS MARKINGS IN MUD AT RUAFA, NORTH SINAI

T

T. Sutton Bowman

HE Wadi el Arish drains between one-third and one-quarter of the Peninsula of Sinai into the Mediterranean. This wadi is dry at ordinary seasons, but conveys seils (flood streams) during the infrequent and heavy rains which visit the region. At Ruafa, a waterhole some 35 miles south-east of the town of El Arish, the wadi passes through a narrow gorge in an outcrop of chalk with flints (of either Lower Eocene or Senonian age), and here the Turks had started to build a dam during the late war.

In the spring of 1924 there were exceptionally heavy rains in Northern Sinai, and the wadi ran in seil for three days, attaining such a volume and violence that the railway, which crosses its mouth near the town of El Arish, was swept away. The writer visited Ruafa in August 1924, and found that a pond, which had been excavated in the east side of the wadi bed to serve as a reservoir in connection with the dam referred to above, had been filled to a considerable depth with the mud brought down by the seil. This mud had hardened and shrunk into blocks of an average width of about 60 centimetres. The cracks between the blocks were from 5 to 10 centimetres, and extended to a depth of about 50 centimetres, the mud being still plastic at that point. It is noteworthy that the blocks were all of an irregular form, and that none approaching the hexagonal shape were seen.

The fine yellow clay composing these blocks was highly calcareous, and was close textured (though porous) and homogeneous to within about I centimetre of the top, at which point the texture became more open, and the clay riddled with small sinuous, flattened, tube-like channels, the length of which lay horizontally rather than vertically. On treatment with hydrochloric acid and washing in water, the clay yielded a small quantity of finely divided vegetable matter. The uppermost 2 millimetres of the clay, forming the actual surface, was covered with small raised irregularities, not more than I millimetre in height, and small holes (like pinholes), the whole presenting a frothy appearance.

As shown in the accompanying photograph, which was taken vertically from a height of about 12 feet, the surface of every block was covered with a perfectly regular series of larger circular depressions, averaging 4 centimetres in diameter and I centimetre depth. These markings were distributed at regular intervals of about 5 centimetres from centre to centre. Mr. T. W. Kirkpatrick kindly obtained some further samples for the writer, and from one of these it was possible to determine that the apparent depressions are, in reality, collapsed bubbles. In one block, two bubbles remain with their upper surfaces still raised some distance above the general level of the block. The "frothy material described above forms the upper skin of the bubble, the interior being quite smooth.

The cause of these markings, which cover an area of some 3000 square metres, is obscure. From inspection of the photograph, it will be evident that their formation was antecedent to the drying and shrinking of the blocks, since the shrinkage cracks are seen to cut across the depressions in many places.

As hail was known to have occurred in Northern Sinai in the spring of 1924, the first explanation that suggested itself was that the markings were the splashes made by the last hailstones of such a shower. This explanation is however hardly satisfactory, since the extraordinary regularity of the markings is scarcely consistent with such a theory. The possibility of the bubbles being produced by gas formed in situ can be dismissed, as there is certainly insufficient vegetable matter to provide the required amount of methane. A gas seepage, either permanent or temporary, from the chalk below however is possible.

Two other simple possibilities presented themselves :

(a) Mechanical Formation of Bubbles in the Mud itself. An attempt was made to test this supposition by puddling a quantity of the material in a large receptacle, and agitating it vigorously with a mechanical stirrer at various stages in the drying of the mixture. A few small bubbles were produced by this means; but they were all of small size and irregularly distributed.

(b) Trapping of Air in already Dried Mud by Water rising from below, the Surface having already been puddled by Rain. The clay is certainly very porous; but experimental testing of this hypothetical explanation would involve the construction of special apparatus, and the expenditure of a considerable amount of time. The time factor being prohibitive, no experiment could be made.

None of the explanations which have suggested themselves seem consistent with the size and regularity of the bubbles as seen in the field. It is possible that some reader of the Fournal has observed similar markings in circumstances which permitted the true cause to be ascertained.

Hoping to find some explanation of these interesting facts before publishing Mr. Bowman's note, we submitted the material to Mr. W. Campbell Smith, of the Natural History Museum, who replies thus :

"I have shown Mr. T. Sutton Bowman's letter and photograph to about half a dozen geologists of various kinds. The only one who has made any