KATAHDIN SYSTEM. 109 



Morrison Pond. This broad plain consists of nearly horizontally stratified 

 sand and gravel, the material becoming finer as we go away from the mouth 

 of the outlet of Morrison Pond. This proves that it is a delta deposited 

 in some body of water. These plains are about 120 feet above the sea, and 

 at the south the sand passes into marine clay, which covers the valley of 

 Union River from this point all the way to the coast. It is therefore evident 

 that the great Katahdin glacial river here emptied into an arm of the sea 

 which extended up the valley of Union River to a point several miles above 

 these plains. But the history does not here come to an end. From near 

 the Morrison Pond outlet a ridge or series of ridges of coarse gravel, cob- 

 bles, and even bowlderets, extends southeastward across the Silsby Plains. 

 These ridges rise above the sun-ounding plain. They are of arched cross 

 section and are clearly of different origin from the plain of nearly horizon- 

 tally stratified gravel and sand which surrounds them. Near the Union 

 River, on the west side of the plain, this ridge of coarse matter is inter- 

 sected by several lower transverse ridges which are parallel with the trend 

 of the valley, and it is also deeply cut through by furrows having the same 

 •direction. Apparently the swift tidal currents as they swept up and down 

 the valley cut furrows thi-ough the ridge, which crossed the valley obliquely, 

 and built up the matter as transverse ridges. 



The facts, so far as known, indicate that the history of this interesting 

 locality is as follows: While the ice was still deep, the glacial river flowed 

 through the Morrison Pond Pass and so on obliquely across the level val- 

 ley of the west branch of Union River, where the Silsby Plains now are, 

 and deposited the ridge of coarse matter. But during the final melting of 

 the ice the sea advanced, and finally covered all the valley to a depth 

 of about 100 feet. But the ice to the north in the Penobscot Valley was 

 not yet melted, and the glacial river continued for a time to pour its freight 

 of sediment into the bay, and the tide carried the finer matter far and near 

 in nearly horizontal stratification. The delta thus formed extended about 1 

 mile north of the mouth of the glacial river and 4 miles south and south- 

 east. While this was going on, the tides, sweeping up and down the val- 

 ley, partially washed away the ridge which had been laid down before, the 

 melting of the ice, cut transverse channels through it, and reclassified the 

 matter. According to this hypothesis, the Silsby Plains consist of an older 

 >osar which was deposited between the ice walls and afterwards bordei-ed 



