280 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
poor in this region, however, that their absence here is of little weight in 
settling the problem. 
The horizontal step scheme, therefore, does not find confirmation in 
the water-plane evidence in the Sudbury valley. Although this scheme 
has been thought to explain the grouping of deltas in the adjoining lake 
basins, it is distinctly the wrong explanation in the case of Lake Sudbury. 
Other Possible Explanations. 
Whatever theory is true for the grouping of the Lake Sudbury deltas 
must account for these facts :— 
(a) The great range of level of the deltas. 
(b) The large number of high-level deltas, 40-60 feet above the level 
of the Morseville pass, and 20-40 feet above Clapp’s 160-foot ‘confluent ” 
water-plane. 
(c) The prebable use of Cherry Brook pass as an outlet, although it 
stands fully twenty feet higher than the Morseville pass at the first- 
opened end of the basin. 
These requirements seem to be met by two' theories, — that of irregular 
fluctuation of lake-level by reason of ice-dams, as already mentioned on 
page 273, and postglacial tilting of the land in such a way as to throw 
the levels of an originally horizontal step-system into confusion. Early 
in the field season my problem was resolved into this, — the collection 
of data by which I might choose between the two theories, ice-damming 
or postglacial tilting. 
The Ice-Dam Theory. 
The condition of delta levels which might be expected as a result of 
ice-dams or icebergs stranded in the outlets of glacial lakes has already 
been mentioned. Because blockades might happen at any time, not at 
regular intervals, the grouping of high and low deltas would be hap- 
hazard rather than systematic. By great ice-dams at the outlets the 
water-level could possibly be raised at times 40-60 feet above the lowest 
notch in the rim of the basin, so that deltas would be built at that 
height and higher cols might perform the duty of the one temporarily 
blocked by ice. One thing should be noted: although lobes might be 
1 A third possible explanation has been proposed by Dr. Clapp in a recent 
paper on Lake Charles, — viz. a condition of extreme irregularity in the melting 
ice, involving a network of little lakelets at somewhat discordant levels. This will 
be considered in a later paragraph. 
