EVOLUTION OF IRONDEOUOIT VALLEY. l35 



away. Their present is therefore but a partial index to their former 

 extent. But the latter item is so vital to an understanding of the 

 subsequent history of the lower valley as to demand a careful in- 

 spection of the evidence. The widest continuous plains to-day are 

 found along the west side of the Bay (^south of the Ridge), where 

 there was and is copious land drainage. Though trenched and 

 dissected again by the same streams that furnished their materials, 

 these particular plains preserve a lobate front for each of the major 

 brooks, with the development of minor gullies in the intervening- 

 notches. There is none of the concave scalloping of their front 

 slopes such as in other portions of the valley signifies under-cutting 

 by stream meander ; on the contrary the scallops are all convex as is 

 normal in a delta margin, from which we conclude that here at least 

 we have a true measure of the original tilling, and that it failed to 

 encroach farther upon the great depression now occupied by the 

 southern half of the Bay. On the east side, between the Float 

 Bridge and the Ridge Road, the conditions are very different ; the 

 land drainage is exceedingly weak, with no mai)pable brooks, and 

 the silt plain is but a narrow wave-built platform constructed into 

 deep water against a steep declevity. But here again meander-cut 

 concavities are lacking, the nearly straight face being instead cpiite 

 regularly corrugated 1)}- rain wash except where complicated by slip- 

 ])ing of the silts down the till slope to which they cling. With so 

 nuicli stufif necessarily expended in vertical construction alone, the 

 meager supply at this j)oint prevented the plain from advancing 

 far into the basin, and once more the present limits appear to mirror 

 quite closely the former proportions of the deposit. In our belief, 

 this section of the bay was never filled by the silts. 



Southward from the Float Bridge the evidence is quite the 

 opposite. All the slopes are clearly erosional. with meander carved 

 walls throughout the re-excavated valley of the main stream and 

 many flat-topped "mesa" reiunants and "sugar-loaves" in the heart 

 of the valley as pointed out by Fairchild.^- The freshly cut edges of 

 the silt strata are visible at many points in the steep banks. All 

 observers from the time of Gilbert have agreed that in this section 

 of the valley the filling was continuous from wall to wall across the 



1:^. \'oL 3 of these Proceedings, p. 237. 



