546 STUDIES FOR STUDENTS 



the upper fifth of the Httoral zone as the greatest possible hmit over 

 which mud-cracks may form. The upper fifth in level may, however, 

 comprise much more than a fifth of the area, since the salt marshes 

 are especially developed near this level. This indicates that the more 

 favorable places for the development of mud-cracks are either those 

 comprising extensive salt marshes, or regions of unusually great 

 tidal range. As an example of the latter may be cited the Bay of 

 Fundy as pointed out by Lyell.^ 



On the borders of even the smallest estuaries communicating with the bay, 

 in which the tides rise sixty feet and upwards, large areas are laid dry for nearly 

 a fortnight between the spring and neap tides, and the mud is then baked in 

 summer by a hot sun, so that it solidifies and becomes traversed by cracks, caused 

 by shrinkage. Portions of the hardened mud may then be taken up and removed 



without injury When a shower of rain falls, the highest portion of the 



mud-covered flat is usually too hard to receive any impressions; while that 

 recently uncovered by the tide near the water's edge is too soft. Between these 

 areas a zone occurs, almost as smooth and even as a looking-glass, on which 

 every drop forms a cavity of circular or oval form, and, if the shower be transient, 

 these pits retain their shape permanently, being dried by the sun, and being 

 then too firm to be effaced by the action of the succeeding tide, which deposits 

 upon them a layer of new mud. 



In connection with fossil rain-prints this calls attention to another 

 factor in the problem of fossil foot-prints and rain-prints, structures 

 often associated with mud-cracks, and that is the necessity of drying 

 and hardening before the next invasion of waters which would 

 otherwise wash out the newly made record. 



Not wishing to draw an artificial distinction, however, as mud- 

 cracks belonging to the littoral zone may be here included those 

 . made from tides of abnormal rise, especially where the water is 

 driven upward by powerful storms. But where flooding of erosion 

 slopes takes place the mud deposited will be ultimately washed 

 away. Where flooding of a river flood-plain takes place, the sea 

 temporarily invades a region which is periodically flooded by fresh 

 water, and therefore mud-cracks in such regions are not distinctive 

 marks of the occupancy of the sea. 



It is seen then that exceptionally high tides are not important 

 as necessarv conditions for the making of mud-cracks. 



I "On l^ecent and Fossil Rains," Quarterly Journal Geological Society, Vol. VII 

 (1851), p. :^9 



