GEOLOGY OF THE PORT LEYDEN QUADRANGLE 49 



that the stream has not yet cut its way through the alluvium and 

 reworked drift filling the valley bottom, and also because glacial 

 striae and kames near the river level have not been disturbed. Thus 

 also the slight trench cut into the Precambrics along here could not 

 have been postglacial in origin. 



There is still the possibility that glacial waters might have de- 

 veloped the terraces, but there is no evidence of any such vigorous 

 water action especially along the higher part of the limestone ter- 

 race where records would surely be left. Even if a large stream had 

 flowed along the ice edge and under the steep front of Tug hill its 

 gradient would have been too low to be compatible with much cut- 

 ting power. No doubt there was movement of water along the wan- 

 ing Black river ice lobe, but the only current of any importance was 

 a northerly one between the eastern edge of the limestone terrace 

 and the ice margin [sec below]. The limestones here are some- 

 what water-worn, but the stream was about 200 feet below the top 

 of the terrace and thus clearly could not have done the work of 

 erosion over the whole terrace. Also the presence of glacial striae 

 on the terrace shows that no great amount of erosion could have 

 taken place there since the ice retreat. 



It seems certain that the lowermost Paleozoic layers must have 

 extended farther eastward, by overlap on the Precambrics, im- 

 mediately preceding the glacial period. This means that Black river 

 was some distance farther eastward and that the western tributaries 

 from Tug hill entered it with lower gradients. As above shown, 

 the lowest sedimentary beds could not have been cut back to form 

 the steep slope now facing Black river in pre- or postglacial times 

 nor were they cut back by glacial waters. Evidently they were cut 

 back by the ice to develop the steep slope. This allowed Black 

 river to shift westward to its present positipn. Thus the slight 

 trench in the Precambrics here could not have been preglacial. As 

 already shown it is clearly not postglacial and apparently it was 

 formed by ice cutting. The concave character of this inner por- 

 tion of the valley is brought out in the figure and strongly suggests 

 ice work. 



The fact should also be considered that we are here dealing with 

 unaltered sedimentaries, with slightly upturned edges, resting upon 

 a rather smooth surface of igneous and metamorphic rocks, and that 

 tlie lowest sediments are weak sandstones and sandy limestones, 

 which greatly favored the stripping off power of the ice. Robert 



