INTERPRETATION OF PHENOMENA AS TO UPPER GREAT LAKES. 75 
water, the vertical walls of granite rising 30 to 100 feet above the water, 
which is in several places 250 feet deep.* 
Scowred boulders.—At several places on the course of the river there are 
intensely bouldery deposits, which appear to be confined to certain lines 
or tracts and probably mark the position of one or more terminal mo- 
raines. These deposits obstruct the old rock channel at certain points, 
turning the river aside in new courses over obstructions and thus pro- 
ducing the lakes; but there are a number of stretches where for several 
miles there appeared to be no boulders nor any drift of the finer grades— 
only bare granite. Some of these places were probably the sites of rapids 
of moderate descent, but it is certain that this was not the case with all 
of them. ‘The presence or absence of rapids in these stretches was not 
evident from any mark made by the rapids themselves, but was inferred 
from the attitude of faint old shorelines on the lake slopes near by and 
from the upper limit of the marks of scour in certain other rapids where 
these marks may be clearly seen. There are several places where boulder 
dams across the channel have been the site of rapids with currents strong 
enough to wash away all the surface material except the boulders, which 
were themselves moved more or less and rearranged. In two of these 
rapids the boulders were worn or scoured into curiously shaped forms 
by the play of very small pebbles and sand upon them, producing pot- 
holesin many. In some cases boulders of gneiss two feet thick were 
bored clear through and turned into ringstones. Boulders showing this 
kind of modification are plentiful at Mattawa and at Des Epines rapids, 
8 milesabove. In two other well marked bouldery rapids no such modi- 
fication of the boulders was found. Both of these, however, lie next 
below lakes, one at the foot of Turtle lake and the other below a lake 
called Pimisi bay. The other two rapids where the scoured boulders 
are found le a short distance below the debouches of streams that 
brought small quantities of sediment down from the highlands to the 
south. The mouth of Boom creek is just above Mattawa and that of the 
Amable du Fond river above Des Epines rapids. The inference is plain. 
Rapids issuing directly from lakes carried no sediment with which to 
wear the boulders, while those that were fed by sediment-bearing streams . 
were supplied with the tools necessary for the work. It would seem 
certain that boring holes in boulders of gneiss in this way must be very 
slow indeed. 
*R. W. Ells and A. E. Barlow: “The physical features and geology of the route of the proposed 
Ottawa canal between the Saint Lawrence river and lake Huron,” Trans. Royal Soc. of Can., 1895. 
Gives a table of altitudes and many soundings. 
7 ¥F. B. Taylor: ‘The scoured boulders of the Mattawa valley,” Am. Joux. Sci., iy, vol. ili, 
Tarch, 
1897. 
XI—Butt. Gror. Soc. Am., Vor. 9, 1897 
