MUD CRACKS ON STEEPLY INCLINED SURFACES 



GERALD R. MacCARTHY 

 University of North Carolina, Chapel Hill, North Carolina 



It is generally accepted that well-developed mud cracks or 

 desiccation fissures are formed only on level surfaces that have been 

 covered by shallow stands of water. Hence the presence of this 

 phenomenon would be supposed to indicate level, low-lying mud 

 flats ; estuarine flood-plain, or playa in origin. 



In the spring of 1922 it was my fortune to observe at Williams- 

 town, Massachusetts, well-defined mud cracks in what I believe 

 to be an atypical position. A small stream had undercut a bluff 

 composed of finely laminated glacial-lake clays. Down the face 

 of this bluff several mud streams had flowed, solidifying before 

 reaching the brook. The surface of these mud streams was seamed 

 with sun cracks which reached depths exceeding 8 inches, and whose 

 intersections produced irregular polygons varying from 6 to 18 

 inches across. Clinometer readings carefully taken on those por- 

 tions of the surface which exhibited the best-defined polygons 

 ranged from 11° to 38°, with an average of about 22°. Some of 

 the best polygons appeared on the steeper slopes. 



Had any one of these mud flows been covered by later deposits 

 and induration taken place, the presence of these sun- cracked poly- 

 gons along the bedding-plane would have unquestionably been taken 

 as evidence that it marked the contact between two horizontally 

 deposited beds of clay. While the above-noted phenomena may 

 not be at all unusual, it would seem that sufficient attention has 

 not been called to occurrences of this type which might lead to 

 serious stratigraphic errors after consolidation of the inclosing 

 sediments. 



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