550 CENOZOIC TIME MAMMALIAN AGE. 



Islands in Barrow Straits, where they are at different heights, to 

 1000 feet above the sea. 



Material of the beds. — The' beds consist of loose earth or loam, clay, 

 gravel, and occasionally large stones. They may be either destitute 

 of lamination, — a very common characteristic, — or finely laminated 

 like the silt or clay of river-flats. They are sometimes partially 

 consolidated, though usually little more compact than the modern 

 alluvium. Some of the layers, especially those of clay, contain hard 

 concretions, in which carbonate of lime is usually the concreting 

 ingredient. The stratification, wherever observable, is horizontal 

 or nearly so, unless disturbed by the fall or undermining of masses. 



Instances occur of contortions in the clayey layers in which the sandy beds 

 above or below do not partake. Such layers of clay, when thoroughly wet, will 

 readily become flexed from any laterally-acting pressure when the other beds 

 will only be compressed ; # and they have been known to be forced out altogether 

 under the superincumbent pressure of a high bank. 



The surface of the upper alluvial plain is often excessively pebbly or stony, 

 because of the removal of the finer soil in which the pebbles or stones were 

 imbedded by the winds and rains. As the beds are soft, they are readily worn 

 away by both causes. In many regions the great plain is changed into a suc- 

 cession of hills and ridges by the action of the rills and streamlets set in motion 

 by the rains, and the surface of each becomes spread over with the stones or 

 pebbles left loose by the process. 



Thickness. — The thickness of the formation, when the river is 

 large and the valley broad, may be thousands of feet ; for, besides 

 rising above the stream, it underlies its bed, filling the valley, what- 

 ever its extent, down to the true Drift, or some other subjacent 

 formation. If, however, solid rocks form the bed of the river, the 

 whole thickness is open to view. All the material of the valley- 

 alluvium belongs properly to the formation of the Cham plain epoch, 

 excepting the portion which has been worked over and brought in 

 by the stream in making the surface of the lower terraces and the 

 modern river-flats. 



Relation to the level of the adjoining river. — There is in all cases some 

 approximate parallelism between the upper level of the formation 

 and that of the bed of the adjoining river, although often obscured 

 by the wear or denudation which has taken place since it was 

 formed. Yet through long distances there is often a gradual and 

 regular divergence from this parallelism, in two ways. 



(1.) There is usually over the continent a slight increase of 

 height in going north. 



On the coast along the southern borders of New England, as at the mouth of 

 the Connecticut, or at New Haven, the height of the upper plain above the river 

 is about 35 feet; at East Hartford, Ct,, 36 niijes north, 40 feet; at East Wind- 



