178 STUDIES FOR STUDENTS 



Shoal breccias are commonly of limestone. Calcareous sediments 

 rapidly harden by cementation, and may be broken into breccia by 

 waves which under identical conditions merely redistribute the 

 grains and particles of unindurated sands and clays. On shoals 

 of calcareous sediments lying partly above and partly below the 

 plane of effective wave-erosion, waves and tides tear up the 

 cemented beds of the elevations and deposit fragments in the 

 hollows safe from further wear. 



In the case of the extensive sheets of brecciated limestone of 

 the Galena and the Niagara formations of Wisconsin, Chamberlin 1 

 has suggested that the tide may have played an important role. 

 Under a large tidal oscillation storm waves may be brought at low 

 tide within reach of the surface of shoals which, except at this 

 brief interval, remains below wave-base. Fragments torn at low 

 tide by storm waves, and the finer waste stirred into suspension, 

 thus have time to settle back together as fragments and matrix 

 of a breccia, and, it may be added, to be further protected by a 

 cover of other sediments before a low tide again coincides with a 

 heavy storm and the process is repeated. 



Strong tides working on shoals are postulated by Lane 2 

 in explaining the limestone breccias of the Salina and Lower 

 Helderberg of Ohio. The prevalence of ripple-marks, mud-cracks, 

 brecciated and conglomeratic layers, leads to the inference of a great 

 flat which seems to have been just awash. "If we imagine tides 

 like those of the Bay of Fundy rushing over this flat, producing 

 this breccia and conglomerate .... we have the conditions 

 of the Helderberg on Monroe deposits." 



In explaining the foundations of Mississippian reefs in York- 

 shire, England, Tiddeman 3 infers local deformations which here and 

 there brought the sea-floor above wave-base. As a result, shoals 

 were formed of wave-plucked angular and more or less rounded 

 fragments on which colonizing corals and mollusks reared their 

 reefs. In a similar way certain Algonkian breccias of Idaho are 

 explained by Ransome and Calkins. 4 "It is supposed that in the 



1 T. C. Chamberlin, Geology of Wisconsin, I (1883), 168-69. 



2 A. C. Lane, Geol. Surv. of Michigan, V (1895), Part II, p. 27. 



3 R. H. Tiddeman, cited by Marr, Quar. Jour. Geol. Soc. London, LV (1899), 330. 



4 Op. cit., p. 38. 



