MEDINA SANDSTONE. 
51 
Sometimes the ridges are much larger than here represented, at others the marking is very 
fine, and barely raised above the general level. The same appearance occurs in the Clinton 
group, often presenting beautiful specimens, like the tracings of a sculptor’s chisel. 
Shrinkage Cracks. 
These marks, as recent productions, have probably fallen under the observation of every 
one, appearing in the clayey beds of shallow pools which have rapidly dried in the sun, 
when the mud shrinks and cracks in various directions. Such marks could never be pro¬ 
duced beneath water, and consequently their occurrence in rocky strata affords a clear 
inference that the surface in question has been once elevated above the water, and subjected 
to atmospheric influence, or perhaps to the sun’s rays. As these are usually produced in soft 
shaly beds, it requires, for their preservation, that the next deposit be of different material, 
and thus casts of the cracks are preserved. Sometimes, however, the cracks themselves are 
preserved in the stone. 
Although I have met with no examples of this kind in the neighborhood of Lockport or Me¬ 
dina, where the ripple marks are so well preserved, yet farther east we find them in great 
perfection. The mass in which they occur is a soft red shale, and is succeeded by a sandy 
deposit. The sand has filled these cracks, and the whole becoming consolidated, the sandy 
layer above separates from the shale below, presenting casts of the cracks in strong relief; 
as seen in the accompanying illustration. 
9 . 
Markings of this kind are not of uncommon occurrence in many of the succeeding deposits, 
and particularly toward the close of the Onondaga-salt group. 
