16 GLACIAL GEAVELS OF MAINE. 



current g-radually loses its motion. Such delta deposits are exceedingly 

 common in Maine. 



Aqueous sediments are termed torrential when deposited by very rapid 

 streams, fluviatile when deposited by ordinary rivers, lacustrine or lacustral 

 when deposited in lakes, marine when in the sea, and estuarine when in that 

 portion of a river subject to the ebb and flow of the tides. While in one 

 sense a portion of the sea, the estuary is inclosed like a river, and therefore 

 its deposits differ from those of the open sea. The water is more or less 

 brackish, and only the remains of animals naturally frequenting such places 

 are found in estuarine sediments. 



The sediment deposited by rains and streams on the land is termed 

 alluvium, and when in the valleys of ordinary streams it is often named 

 valleij drift. Observations in all parts of New England show that a very 

 large amount of alluvium was deposited in the larger valleys at or near the 

 close of the Glacial period. So characteristic is this alluvium that the 

 period has sometimes been termed the Valley Drift period. 



The principles enunciated above enable us to estimate approximately 

 the velocities of the rivers at the time the valley drift was deposited. The 

 size of the fragments contained in the valley drift is such that the velocity 

 necessary to transport them is generally less than 4 or 5 miles per hour, 

 but among the hills it may have reached 8 or 10 miles. This refers to the 

 velocity near the bottom of the streams. The slope required to jDroduce 

 these velocities varies according to the breadth and depth of the stream, etc. 

 The viscosity of water is so small that only very swift currents can 

 transport large stones and bowlders up and over a steep obstacle. The 

 water at the bottom is embayed or dammed by the obstacle, so that the 

 rest of the stream flows over and aroimd the embayed water as well as the 

 obstacle. Hence, the mutual adhesion of the pebbles of a gravel bank is 

 often sufficient to protect the bank from erosion when the velocity of the 

 current is far greater than would otherwise suffice to transport the pebbles. 

 The pebbles become wedged together like paving stones, so that they can 

 not be moved without friction, and they, resist erosion by swift currents as 

 the gravels of the seabeach resist the surf. 



A practical application of these principles involves the vexed question: 

 How can we account for the presence of stones several inches in diameter 

 in the midst of fine sand and clay ? It has been usual to refer the cobbles 



