SUEEACE DEPOSITS. 109 



water does not accumulate in the Bluff Deposit, it should not 

 be inferred that it is not always moist, for it always contains 

 sufficient moisture to give it the ordinary coherence of common 

 soil even at the surface; and crops growing upon it suffer no 

 more from drought than they do upon any other soil. As a 

 soil, it has the additional advantage of being constantly and 

 completely under drained. 



Physical Features of tlie Region. Hitherto the Bluff 

 Deposit has been spoken of in relation to the character and 

 aspect it exhibits in and near the range of bluffs that border 

 the flood-plain of the Missouri river. It now remains to 

 speak of the physical features of the region occupied by the 

 deposit, in a more general way. 



In this connection it will be necessary to give some account 

 of the geographical extent of the region it occupies, together 

 with its known thickness, when that has been ascertained. 

 The limits of this area have nowhere been ascertained with 

 absolute accuracy, but these may doubtless be defined with 

 as much precision as we can define the exact limits of any 

 of the formations which underlie the drift. The difficulty of 

 defining its limits arises from its gradual passage into the 

 fine top soil of the drift, or rather from the fact that the finely 

 comminuted, humus-stained soils of both deposits are so 

 nearly alike in aspect when unbroken by natural or artificial 

 excavation, that it is difficult to say where one ends and the 

 other begins. However, the following outlines are assumed 

 to be not far from its eastern and northern limits in Iowa : 



Commencing at the southeast corner of Fremont county, 

 follow up the watershed between the East Mshnabotany and 

 the west Tarkeo rivers to the southern boundary of Cass 

 county, thence to the centre of Audubon county, thence to 

 Tip-Top station on the Chicago and Northwestern railway, 

 thence by a broad curve westward to the northwestern corner 

 of Plymouth county. The last named point is probably 

 very near the most northerly one to which the deposit any- 

 where reaches, certainly the most northerly one to which it 

 reaches in Iowa. Less is known of its nothern limits in 



