UPPER STRATUM OF VALLEY DRIFT. 487 



As elsewhere noted, these fluviatile deltas can be traced in all the 

 larger valleys. The delta of the Androscoggin reaches to the sea, or 

 nearly, as ought to be the case where a large stream continues to poiir 

 sediment into the sea during the whole time of the retreat. The delta of 

 the Kennebec covered not only the basal clay of the valley with coarse 

 gravel and cobbles from Bingham for many miles southward, but also all 

 the fossiliferous clays from Norridgewock south to a breadth of several 

 miles. From Madison south the delta consisted of sand; northward it 

 became coarser. The delta sand is not traceable south of Waterville. 

 The fluviatile delta of the Penobscot is indistinct south of the mouth of 

 the Piscataquis River. I have not been able to trace definitely the clays 

 which naturally belong to a fluviatile delta of sand, but undoubtedly the 

 finer sediments were swept out to sea and helped form the upper or 

 sparingly fossiliferous clays. 



5. South of where the fluviatile deltas of the Kennebec and Penobscot 

 rivers disappear as broad sheets there are low plains or lateral valleys which 

 would be covered by sea Avater up to the time when the sea had nearly 

 sunk to its present level. If these rivers continued to bring down the same 

 quantity of sediment as formerly, I do not see why the flu^datile deltas 

 should not be prolonged all the way to the sea, or at least they should 

 spread laterally into these broader bays of that time. 



Various reasons can be assigned for these deltas failing to be extended 

 all the way to the sea. Thus, as the ice receded toward the north a larger 

 proportion of the sediment might be dropped at a distance from the sea. 

 The supply of glacial sediments would diminish as the ice melted. The 

 flow of water may have diminished as the elevation advanced. As the 

 gradients became steeper the sediment would be carried out farther to sea 

 and would tend less to spread into the lateral bays. Parts of deltas may 

 have disappeared by erosion. The net result was that the deltas were 

 narrow, no longer extended back from the rivers, and are hardly dis- 

 tinguishable from the flood plain. 



The existence of Merrymeeting Bay has a bearing on the history of 

 both the Kennebec and Androscoggin rivers. Into this large lake-like body 

 of water both these rivers flow. Both have formed delta flats near where 

 they enter it. If there had been any such transportation of sediments 

 when the sea stood, say, 30 feet above its present level, as took place 



