532 STUDIES FOR STUDENTS 



become completely obliterated. This, therefore, may be a test as 

 to their former presence or absence. 



A pure clay, slowly subsiding from quiet waters, and wet suffi- 

 ciently long to become compact upon drying, would retain its mud- 

 cracks upon rewetting, either by rain previous to flooding or by the 

 flood waters themselves. Such a clay, on account of its tenacity, 

 resists erosion even by quite rapid currents, as is seen from the 

 presence of occasional areas of sticky blue mud on relatively shallow 

 and open parts of the coastal shelf. In case such a sun-baked clay 

 is covered and its cracks filled by a similar layer, it should retain a 

 clear record of the cracks, provided it possesses a well-defined bedding 

 cleavage, since such a cleavage will be interrupted at the margins of 

 the cracks. Such pure and massive clay deposits form, however, 

 the rocks most susceptible to dynamic metamorphism, and a pressure 

 cleavage, even if developed upon the bedding planes, would tend to 

 mask any previous interruptions. Frequently, however, such clay 

 deposits from standing water will be interstratified with more or 

 less arenaceous deposits swept along the bottom by the rising floods. 

 Such sandy wash filling the cracks of the previous clay layer would 

 give a persistent record to the buried mud-cracks. If the sand be 

 sufficiently coarse not to shrink markedly upon drying or swell upon 

 rewetting, the combination of the two kinds of laminae should give a 

 maximum opportunity for the complete preserval of mud-cracks, 

 footprints, raindrops, and other surface markings. It is noteworthy 

 that this is the typical nature of those beds in the Triassic formations 

 of the Connecticut valley which have preserved such a magnificent 

 record of mud-cracks and footprints. Large portions of these for- 

 mations, however, consist of rather massive sandy shales or arkoses, 

 and in these the writer has not noted the occurrence of mud-cracks. 



On the larger river flood-plains, such as that of the Mississippi, 

 the soil survey of the Department of Agriculture has established 

 three principal types, grouped under the Yazoo series and seven 

 miscellaneous types. Of these ten types only two are clays. The 

 Yazoo clay — a heavy, drab clay loam — occupies low areas back 

 from the low, flat ridges which form the front lands near the stream 

 courses. It is a frequent type of soil. The Sharkey clay is a 

 stiff, impervious clay occupying the lowest portions of river bottoms 



