118 PHYSICAL GEOGRAPHY. 



them are similar in character to the Bluff Deposit, "but having 

 been derived from different materials, they are never pre- 

 cisely like it. They always contain a considerable amount 

 of clay, which the Bluff Deposit contains almost none at all 

 of. Others are remains of marshes and not lakes, or at least 

 of marshes which may have existed upon the borders of 

 lakes; for the waters in which these deposits took place, are 

 believed in every case to have been lake-like expansions, or 

 comparatively still portions of the rivers upon the borders of 

 whose valleys these deposits are now found. 



Those that are more distinctively lacustral in their character 

 are composed, in varying proportions of fine silicious and 

 clayey silt, such as would be deposited in comparatively 

 still waters. They are found upon the sides of the valleys, 

 where they show indistinct traces of stratification when they 

 are freshly excavated. Those of this character thus far 

 observed, have been found almost alone upon the valley sides 

 of the Mississippi, and almost always near its lower part, 

 or not far above the level of the present flood-plain. 



Traces of such terraces are to be found in almost all cases 

 where the valley of a creek or small tributary comes into the 

 flood-plain of the great river. These, however, can hardly be 

 said to be parts of the proper river terraces which they very 

 much resemble, the latter being abandoned flood-plains, and 

 the former a deposit of fine silt in waters stiller than those 

 which brought it down from the uplands. An interesting 

 example of such a silt-like deposit exists within the city of 

 Burlington, having been produced by Hawkeye creek when 

 the Mississippi occupied a level higher by forty feet than it 

 now does. Much of the deposit was removed by the deepen- 

 ing valley of the creek, and much has lately been removed for 

 railroad purposes from the vicinity of Fifth and Market 

 streets, where it was once finely exposed by the artificial 

 excavation and seen to rest upon the drift. 



Such of these deposits as partake more of the character of 

 marsh accumulations are found in somewhat similar positions, 

 but all seem to have taken place at an earlier period in the 



