CLAY GALLS; CLAY BOULDERS 711 



the seashore. Only under exceptional conditions can we expect 

 the preservation of extensive mud-cracked surfaces of the sea- 

 shore. Moreover, the formation of great littoral deposits must 

 be accompanied by a subsidence of the sea-floor, and a consequent 

 landward transgression of the seashore with its attendant phe- 

 nomena. The migration of the shore zone would thus bring about 

 a transference of the zone of mud-crack formation, so that we 

 could hardly expect to find a thick marine formation with repeated 

 horizons of mud-cracked layers unless we assumed that the subsi- 

 dence was so gradual a one that deposition kept pace with it. 



7. Clay Galls (Thon-gallen). When the mud layers on 

 the playa surface or on the river flood plain are very thin, drying 

 will cause them to curl up into masses resembling shavings. Such 

 curly mud shavings occur on river flood plains exposed to a hot sun, 

 and they may also under favorable conditions be formed on the 

 seashore, as in the case of the coast of the Red Sea (Walther- 

 23 '■847). When thoroughly dry these shavings may be blown by 

 the wind into neighboring sand dunes in which they become buried. 

 Subsequent softening of the clay when the sand dune is saturated 

 with water, as in the rainy seasons, will bring about a compression 

 of the clay-shaving into a thin, flat pellet of clay which will lie 

 embedded in the sand parallel to the stratification. Such clay pel- 

 lets, called Thon-gallen, or clay galls, are common in sandstones 

 of subaerial origin, especially in red sandstones. They may, indeed, 

 be regarded as practically positive evidence of a subaerial origin 

 of the rock containing them, though these rocks may be seashore 

 dunes or formed far inland. 



8. Clay Boulders. Clay boulders formed of plastic clay 

 rolled about by the waves are not uncommon occurrences on the sea- 

 shore. They have been recorded by Walther (2^:84/) and by 

 O. Fraas (10:2//) from the coast of the Red Sea. On the coasts 

 mentioned they represent the clay deposited during the previous 

 high tide, which on exposure at low tide dries and breaks up into 

 fragments. These are rolled into balls by the returning tide and 

 incorporated in the later sediment, where they have the appearance 

 of concretions. Examples of such structures seem to occur in the 

 Devonic calcilutytes of Michigan, where rounded balls of a darker 

 color are included in lighter bedded deposits of similar character. 



The author has observed the formation of clay boulders on the 

 coast of Scotland. Here fragments of glacial clays broken from 

 the clifi:'s are rolled about by the waves and fashioned into pebbles 

 and boulders of elongate but well-rounded outline. Where these 

 are rolled over a pebble beach, the hard pebbles are pressed 



