134 ROCHESTER ACADEMY OF SCIENCE. 



all sides by man or stream cutting, to wonder whence all this stuff 

 could have come. Though deeply dissected by numerous gullies on 

 every side the upper surface of these plains still presents to the 

 eye a perfectly straight and horizontal line of great beauty. Yet 

 the relation of these flats to the Iroquois lake plane would appear to 

 vary at different points. In the open northern portion of th^ Pitts- 

 ford embayment they evidently mark "wave base" but as each suc- 

 cessive narrowing of the bay diminished the wave force the depth of 

 wave base below water-level must have correspondingly de- 

 creased. A critical observation bearing on this was possible in 1909 

 in the fresh cuttings for the state road on Atlantic Avenue, where, 

 along the east ascent from the valley, the upper ten feet or so of 

 the silts showed a conspicuous flow-and-plunge structure, sharply 

 separated from the uniformly horizontal laminae below. This seems 

 to indicate that the silts were here able to accumulate above the orig- 

 inal wave base and by their own slow encroachment to retard the 

 wave activity. We may safely assert that the surface of the silt 

 filling rose slowly southward as in the present Bay (which, how- 

 ever, from its smaller size and deep implantation, is a comparatively 

 placid body of water) until it surmounted the lake level and became, 

 first a marsh and then a grade-plain (flood plain) of the river. Fair- 

 child has shown^^ that the original grade of this plain had some six 

 feet per mile of southward rise in the most exposed portion and one 

 is tempted to compare this figure with the exactly similar gradient 

 of the submarine silt plain for one hundred miles ofT New York 

 harbor. But these figures of Fairchild take no account of the sub- 

 sidence that these soft silts underwent when the sustaining and per- 

 meating lake waters were withdrawn. (Sec discussion of this 

 farther on.) As shown later there are many evidences, especially 

 the course of Allen's Creek after it enters the main valley, that point 

 to sub-aerial construction of the silt plain as far north at least as 

 the Rich's Dugway (Zarges ]\lill), though possibly this was true 

 only of the final stages when the waters of Iroquois had already 

 commenced to lower. 



These silt plains are now in the process of removal, a process 

 that has been going on uninterruptedly since Irocjuois was drained 



11. Proc. Roch. Acad. Sci. 3: 2.SS. 



