RILL-MARKS; MUD CRACKS 709 



characteristic depressions, or the water falling over the obstruction 

 will gully the surface for a short distance below. Examples of 

 these are found in the Medina sandstone. 



6. Mud-cracks, Sun-cracks or Desiccation Fissures. 

 When lutaceous deposits are exposed by the retreat of the tide,, or 

 by the shrinking- or disappearance of a playa lake or a pond, or by 

 the uncovering of the flood-plain of a river, the drying which they 

 undergo will result in the formation of polygonally arranged cracks, 

 and a gentle concaving of the upper surfaces of the polygons thus 

 bounded. When the desiccated layers of mud are thin, they will 

 often curl up like wood shavings, and may be blown away by the 

 wind. When on reflooding of the surface or by the deposition of 

 wind-blown material, these cracks are filled in by deposits of the 

 same or different material, the polygons will remain more or less 

 perfectly outlined. This desiccation fissure, sun-crack, mud-crack, 

 prismatic, or paving block structure, as it is variously called, is 

 found not only in clayey rocks, but also in fine calcilutytes. like 

 those of the Helderberg mountains (Rondout waterlimes), the 

 Solnhofen beds (Marsh), the Cincinnati limestones (Perry-i8), and 

 in many other lutaceous deposits. They testify to the exposure 

 of these deposits before they were solidified. (For a full dis- 

 cussion of this subject see Barrell-i.) 



Playa Surface. Taking the areas of mud-crack formation in 

 the order of their magnitude, the playa surface would probably 

 stand first. Here the entire surface for hundreds of square miles 

 becomes mud-cracked, often to considerable depth, on the complete 

 drying up of the temporary playa lake. Here, too, the conditions 

 for the preservation are most favorable. Not only is the exposure 

 a long one, often the greater part of the year, or for many years, 

 and for much of the time to intense heat, but the chances of proper 

 burial are much greater. Wandering sand dunes may thus preserve 

 the record, dust deposits may fill the fissures, or, at the next flood, 

 sands or mud may be swept into them. In fact, the playa or takyr 

 seems to be the ideal surface for mud-crack record, and one is 

 tempted to refer most mud-cracked strata to such an origin. Cer- 

 tainly where fossil mud-cracks penetrate a formation to the depth 

 of ten feet, as is the case in the upper Shinarump ( Jura-Triassic) 

 shales of Utah (Gilbert-ii : 9), it is difficult to believe that they 

 could be formed under other conditions than those permitting pro- 

 longed exposure such as is found only in the playiis of the desert, 

 where ten years or more may elapse between rainfalls. 



Permanent Lake Surface. Much less extensive, and of minor 

 significance, are the sun-cracked areas which come into existence 



