108 A MANUAL OF TOPOGRAPHIC! METHODS. 



generalizations. This will enable him to decide what details should be 

 omitted and what ones preserved, and, where details are omitted, what to 

 put in their places in order to bring out the dominant features. 



It is not possible to define the degree of detail which the maps should 

 represent. The limit commonly given — that is, the limit imposed by the 

 scale of the map — is not always the best. In representing country which 

 has little plan or system, such as moraines or sand dunes, it is well to work 

 in as much detail as the scale will bear. But where the country shows a 

 system in its structure to which the minor detail is subordinate, the omission 

 of some of this detail may give greater prominence to the larger features. 

 The amount of detail thus omitted must necessarily be left to the judgment 

 of the topographer, but no more should be omitted than is necessary to 

 give full expression to the general features of the country. 



ORIGIN OF TOPOGRAPHIC FEATURES. 



As an aid in the interpretation of the various topographic forms which 

 present themselves, the following brief discussion is appended. 



Topographic features originate from a variety of causes and are modi- 

 fied by many agencies. They are formed by uplift from beneath, of great 

 or small extent, They are formed by deposition from volcanoes, glaciers, 

 watei - , and the atmosphere. They are formed or modified by aqueous and 

 ice erosion. They are modified by gravity. 



These are the principal agencies in producing topographic forms as we 

 see them to-day. These forms are only in rare cases the work of a single 

 one of the above agencies; generally two or more have taken part in pro- 

 ducing the present condition. Of all these, aqueous agencies are by far 

 the most potent. Their work is seen in nearly all topographic forms, while 

 in those of great age their action has been so extensive as to mask or oblit- 

 erate all superficial traces of the action of any other agency. 



The internal stresses of the earth, however produced, have resulted 

 in raising certain portions of the crust and depressing others. Commonly 

 these movements have been slow and of great duration. Some of them 



