284 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Since in the lakes of eastern Massachusetts there is not usually a 
single water-plane, but several, it is worth while to consider how a set 
of these will look when tilted. In their original position, as we have 
seen, the deltas occur in groups, each group marking a horizontal water- 
plane, and standing higher than the group that lies icewards from it. 
3etween the brow of any sand delta of the higher group and that of any 
delta of the lower group, there is a difference of elevation equal to the 
difference of the lake-level in the two stages. The outlet for each water- 
plane has its position just below the lobe-brow levels, and is likely to 
occur at or near the outer extremity of the water-plane. Now, if the 
region be tilted, each water-plane is tilted as already discussed, and the 
brows of the plains in each group will therefore indicate a rising of 
the water-plane icewards. The original drop from one water-plane to the 
next can no longer be measured by the difference in altitude between 
the lobe brows of any two representative deltas, for the original differ- 
ence of elevation between them has been modified by the tilt. It may 
be that some of the deltas of the earlier, higher water-plane now stand 
actually lower than some of the deltas of the later, lower water-plane. 
The sand deltas, then, after tilting, should ascend icewards, but with a 
drop in elevation at each new water-plane. The relation of the altitudes 
of outlets would likewise be disturbed. Take, for example, any two adja- 
cent outlets controlling the water-planes of two neighboring groups. 
Before tilting, the iceward outlet is the lower. The effect of tilting 
is to raise the lower outlet toward, to, or even above the level of its 
neighbor. The relative altitude of the two after tilting will depend 
on (a), the original difference of altitude (4), the distance apart of 
the outlets, measured in the direction of the tilt, and (e), the rate of 
tilting. 
If the tilting of the extinct lakes was a simple uniform one, taking 
place after all the deltas had been built, the tilted water-planes should 
of course all have the same slant; in other words, the water-planes 
should in that case be parallel. But if the tilting was already going on 
during the retreat of the ice and the building of the deltas, the older 
water-planes at the further end of the lake would have been tilted more 
than the younger water-planes. Diagrammatically, the two cases would 
appear like Figure 5. Gilbert speaks of the application of this principle 
to the lakes of the Ontario basin (603). 
In summing up the condition of lobe brow and outlet levels for a lake 
which has gone through the normal ice-front history, plus postglacial 
tilting, this may be said: the sand plains should be resolvable into 
. 
