GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 283 
The Probable Effects of Tilting on Temporary Lake Features. 
Taking the typical temporary lake, with its successively lower outlets 
and sand plains in the direction of ice-retreat, let us see how tilting will 
rearrange the levels. 
To take the simplest case, — suppose that a single water-plane, marked 
originally by a set of sand deltas with brows at a common level and an 
outlet at a level slightly below them, is tilted evenly, with a relative ele- 
vation of the iceward side (see Fig.3). After this movement, just as 
before, all the brows of the sand deltas lie in a single water-plane; but 
the plane is no longer horizontal, —it slants away from the ice. The 
sand deltas nearest the ice-field have been lifted higher than those far- 
thest from it, the brows of the two deltas at these two extremities of the 
lake measuring the greatest difference of elevation of the inclined water- 
Direction of Ice Retreat ——> 
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Ficure 4. 
Diagram showing how postglacial tilting, under certain conditions, may place a delta 
below the level of the corresponding outlet col. In the upper diagram, the ice-sheet has 
melted back more rapidly along the rim of the basin than in the basin itself; conse- 
quently the first delta built—the one on the left —belongs to an outlet col on the 
iceward side. With further retreat, a second delta has been built, on the right. In 
the lower diagram, the region has been tilted; and the first-formed delta has thus been 
brought below the level of the outlet. 
plane, and the deltas intermediate in position showing intermediate 
elevations of a measure proportionate to their distances from the two ex- 
tremities. The outlet associated with these deltas, originally at or some- 
what below the horizontal water-plane, will of course now be found at or 
alittle below the slanting water-plane, as shown in Figure 3. If its posi- 
tion is at the further extremity of the lake (relative to the ice), or if it is 
at the further end of the water-plane for its stage, its level will be the 
lowest point marked on the water-plane ; but in case the outlet was on 
the side of the lake, and icewards from the first delta built at that water- 
plane (a condition of things made possible by an irregular ice-front), its 
actual altitude measured after tilting will be higher than that of the 
sand delta whose level it once controlled (see Fig. 4). 
