762 THE JOURNAL OF GEOLOGY. 
the level of its scour. The contrast in size of preglacial and 
present valleys in such cases is not great, though the work 
accomplished in removing the glacial deposits is no greater than 
in cutting the narrower valley in the rock, shown in the figure. 
In places a preglacial valley is found to carry shelves of con- 
siderable breadth, now concealed beneath the present valley bot- 
tom, thus at Quincy and St. Louis a rock shelf extends entirely 
across the present bed of the Mississippi, with an altitude 60 
feet or more above the deeper portion of the valley. The fol- 
lowing tabular statement of data of the two bridges in St. Louis, 
furnished by Robert Moore, C.E., serves to show the existence 
of such a shelf at one bridge and its absence at the other. The 
Eads bridge is about three miles below the Merchant’s bridge. ° 
Its west end is at the west bluff, while the west end of the 
Merchant’s bridge is one-half mile from the west bluff. The 
shelf, therefore, extends at least one-half mile further into the 
valley than the length of the bridge. 
ELEVATIONS OF ROCK FLOOR AT ST. LOUIS BRIDGES. 
Eads Bridge Merchant’s Bridge 
IdeGl IR@ele IPieir 1, West! SNORE osonc00 ccc 375.71 feet 341.33 feet 
BedPROCkeRier2).a na caer ook eee 341.91 “* BABS 
IBedeRock Pier ais, aca aiden eae ne AOZi7gn BRO) 
Ip@Gl IR@Ele IPE A, IWASE SiMORs ono ocea sacs BAO 234.90 “ 
Id seiieerane Imig WHAWE sooo so 0000 boo couo‘e 420.29 “ ABB, ( P)) % 
leseiieeiane lowe WALER. os 00 pono oc esoaccoaue BHSOy 380.50 “ 
It is probable that such shelves as these are remnants of an 
old valley floor. Their full breadth is not known, hence we 
cannot judge whether they are mere fringes on the border of the 
deep channel, or are of such breadth as to reduce greatly the 
width of the deep channelyeiive datalstronmythe mS ramelecomis 
bridges cover only about one-tenth, while those of the Quincy 
bridge cover one-sixth the breadth of the preglacial valley. If 
the shelves greatly reduce the width of the valley, their bearing 
upon the valley’s history —its stages of uplift, etc—may prove 
to be of considerable importance. It is as yet not certain 
whether the valley was deepened regularly during continuous 
