10 FOURTH REPORT — 1834. 



deeper; the Mississippi, at New Orleans, being above 100 feet 

 deep, which depth it preserves to the mouth of the Missouri. 

 Mobile Bay is crossed by a bar, having only 10 feet of water, 

 and the bar of the Altamaba of Georgia has 14 feet, which is, 

 perhaps, about the average depth to be found at the entrance 

 of most of the southern rivers of the Atlantic coast. 



Alluvial Terraces. — Besides the alluvial flats which border 

 so many of the rivers at an elevation of only a fe^v feet above 

 the tide, and which may have been formed during the present 

 relative level of the land and sea, there are plains of another 

 class, which often occupy the sides of the valleys in terraces 

 more remote from the rivers. This common feature on many 

 of the rivers of the United States, I mention not only from my 

 own observation, but on the authority of various works, as 

 Stoddard's Sketches, Drake's Picture of Cincinnati, Dar- 

 by's Louisiana, and Professor Hitchcock's Report on the 

 Geology of 3Iassachusetts ; some of them mentioning two, 

 three, or even more of these river terraces. The latter author 

 thus describes them on the Connecticut river in Massachusetts: 

 " If we start from the edge of the stream at low water, and 

 ascend a bank of 10 or 15 feet high, we shall come upon an 

 alluvial meadow, which is frequently overflowed, and is conse- 

 quently receiving yearly deposits : this may be regarded as 

 the lowest terrace. Crossing this, we ascend the escarpment of 

 a second terrace, .30 or 40 feet in height, which may be seen at 

 intervals on the same level on all sides of the meadow. This 

 second terrace is rarely very wide in any place, and seems to be 

 only the remnant of a meadow, once much more extensive, 

 which has been worn away. Ascending from this 40 or 50 

 feet up another escarpment, we reach the plain that forms the 

 bottom of the great valley of the continent : this constitutes the 

 upper terrace." He adds, that terraces, more or less distinct, 

 exist on almost every stream of considerable size in the State, 

 wherever the banks are low enough to admit of alluvial flats. 

 Professor Hitchcock imputes these terraced valleys to the sud- 

 den bursting of the barriers of a lake or pond through which 

 the stream flowed, or the sudden removal of an obstruction in 

 the river, by which it cut a new channel into the soft soil above 

 the obstruction. I -would beg leave to suggest, however, whe- 

 ther, in the case of so many successive terraces, such an ex- 

 planation is not rendered improbable, from the difficulty of 

 imagining so many debacles taking place in succession upon 

 the same river. The circumstance that nearly all our river 

 valleys which have the structure described, occur in districts 

 where the rivers could ne^er have been crossed by ridges of 



