242 GLACIAL GRAVELS OF MAINE. 



to the lake the material of this plain is very coarse, containing great numbers 

 of cobbles, with bowlderets and some bowlders. The surface is here very 

 irregular, and the gravel consists of a series of reticulated ridges inclosing 

 kettleholes and basins of various sizes, some of them occupied by lakelets 

 and peat swamps. The Maine Central Railroad was originally constructed 

 across one of the peat swamps. The peat soon sank under the weight of 

 the roadbed, showing that the peat overlay a lakelet. The chasm was then 

 filled up by an embankment of gravel, 85 feet above the top of the water, 

 which stood at the same level as the lake. A depression 95 feet deep is 

 found on the bottom of the lake a few rods north of the shore at the 

 south end of the lake. It is surrounded on all sides by much shallower 

 water, and is probably a kettlehole. The water at the south end of the 

 lake is from 20 to 40 feet deep except at this depression. No rock in 

 place appears anywhere near the -south end of the lake nor along a line 

 extending southeast from this point. 



The most probable interpretation of these facts is this: In preglacial 

 time the region where Sebago Lake no-w is was di'ained by a valley which 

 extended from the foot of the lake southeastward to the Presumpscot 

 Valley near Saccarappa. In late glacial and early postglacial time this 

 valley was tilled by till, glacial gravel, and sedimentary clay to a depth of 

 100 feet or more. After the final melting of the ice the water found the 

 old drainage valley effectually dammed, and it filled up the basin till it 

 began to ovei'flow 7 miles northeast of the old channel The Presumpscot 

 River (the outlet of Sebago Lake) flows over a rock bed, showing a constant 

 succession of rapids and waterfalls all the way from Sebago Lake to near 

 Saccarappa. This indicates that it is a recent channel for the main stream, 

 though in preglacial time this valley was occupied by a branch of the main 

 stream. Sebago Lake would be about 100 feet lower than it is but for the 

 plain of glacial gravel at its south end, and Avould be greatly reduced in 

 size. Portland owes the convenience of its water supply to this same dam 

 of glacial gi-avel.^ 



The plains of glacial gravel that border the lake vary from 10 to 40 

 feet in depth, except at the south end, where they exceed 130 feet. The 



'Since the above was written I have discovered that the gravelly nature of the southern 

 boundary of Sebago Lake attracted the attention of Prof. C. H. Hitchcock; see Preliminary Eejiort 

 upon the Natural History and Geology of the State of Maine, p. 288, 1861. 



