OTTAWA VALLEY CONCRETIONS 627 



surface of the hill slope. Workmen with many years of experience in 

 these pits report never having seen them in the clays which have been at 

 a much greater depth nor in the clays of the upper pit, which are less 

 silty. This distribution the writer's own observations appear to confirm. 

 The relation of these concretions to the inclosing clays is indicated in 

 plate 10, figures 2-5. The limitation of these concretions to beds within 

 a few feet of the surface indicates that their development occurs in the 

 path of waters moving down the surface slope and in the superficial zone 

 just below the surface, where fluctuations of temperature and water 

 supply are greatest. Concretions are most abundant within or near the 

 zone which is subject to freezing and thawing during one-half of the 

 3'ear and alternate desiccation and soaking by rains throughout the other 

 half. The stimulation of the movement of ground-water resulting from 

 these climatic agencies is doubtless the most potent factor in the forma- 

 tion of the hard rugose claystones of these beds. 



Quirke, in describing this type of concretion from the Pleistocene clay 

 of the Espanola district, Ontario, states that he does not consider them 

 true concretions, but "rather to be the indurated remains of the viscous 

 fluid, which was covered by fine materials." *^ Hardened clay pebbles 

 are formed in abundance along the banks of the Ottawa Eiver and may 

 be preserved for a considerable period, but none of the hundreds of such 

 pebbles which the writer has examined shows any of the peculiar mark- 

 ings and erratic incised lines on their surface found on the concretions 

 of these upper clays and shown also in Quirke's illustrations. These 

 peculiar features the writer considers characteristic of this class of con- 

 cretions and impossible of formation and preservation on dried clay 

 pebbles. 



Associated with these mm^lehor or rough-surfaced forms another kind 

 of concretion occurs, if this term may be applied to the slightly indurated 

 cylinders of clay which mark the course of many small tree roots which 

 penetrate these beds. The difference in both hardness and composition 

 between these and the inclosing clay is very slight. A color contrast, 

 due presumably to a trace of iron, clearly outlines these structures, which 

 may be considered a preliminary or antecedent phase of one type of 

 cylindrical concretion in which an organic agent serves as the original 

 stimulus to development (plate 11, figures 11, 12). 



The higher clay beds at the Billings Bridge brick-clay pits, the O'Reilly 

 pits, and elsewhere show still another type of non-indurated concretion 

 in which the hardness, though greater in some cases, differs little, if any, 



^« T. T. Quirke : Can. Geol. Surv. Mem. 102, 1917, p. 56. 



