140 



E. M. KINDLE 



writer an opportunity to see several acres of the calcareous mud 

 which forms much of the bottom of the northern part of BiscayneBay 

 exposed to subaerial conditions. North of the causeway to Miami 

 beach six or eight acres had been covered by gray calcareous mud 

 pumped from the shallow sea bottom by hydraulic dredges. The 

 resulting expanse of mud cracks furnished an exceptional oppor- 

 tunity for observing the kind of mud cracks which this type of 

 sediment will yield (Fig. i). 



These mud cracks show a considerable range in the size of the 

 polygons. Over most of the area the polygons were from 6 to 15 



Fig. I. — Mud cracks in calcareous mud, Miami, Florida. Note terrace-like 

 margins of polygons. 



inches in diameter, with cracks separating them which were mostly 

 from 2I to 3 inches wide, but with a minimum and maximum range 

 of from I to 4 inches in width. The depth of the cracks generally 

 ranged between 8 and 15 inches. One part of the area was noted 

 where the polygons ranged between i and 3^ feet in diameter with 

 the separating cracks averaging 3 inches in width (Fig. 2). These 

 large polygons showed none of the tendency to split up into laminae 

 seen in some of the smaller ones (Fig. 3). The margins of the poly- 

 gons of Figure i show clearly three or four laminae exposed, the 

 unequal rates of shrinkage having developed terrace-like margins 

 along the fissures. The different-sized polygons noted in different 

 parts of the Miami mud cracks probably correspond to differences 

 in the desiccating power of the sun when they were formed. A high 



