METHOD ADOPTED 



Graphic representation of an irregular solid is difficult for all but the simplest forms. In 

 this case it is necessary to represent perhaps up to a dozen interlocking solids which are 

 most irregular. In the case of Long Island, several methods of doing this have been tried. 



Geologists customarily map the area covered by a given stratum and then add a few 

 vertical sections (profiles, from which the relief of the surface may be inferred) . This method 

 gives good results in proper cases, but is not suitable for showing a very irregular surface. 



Veatch (9) used contours to show parts of the rock surface and the Lloyd sand. These 

 have been very useful and even rather prolonged extensions of them have met the test of 

 well sinking in good shape. 



Crosby (11) drew a large number of sections across the island, projected on them the 

 logs of the nearby wells and then drew profiles of all the strata in which he was interested. 

 These sections have been most useful. The projection of well logs on to vertical planes, often 

 remote, is intrinsically unsound unless the tops of all the strata are planes with the same strike 

 and sections are taken in the direction of the dip. Any other condition may lead to gross 

 distortion. 



In this Bulletin standard engineering practice has been followed and contours have been 

 sketched for the top surface of each aquifer and geologic formation. Such maps give the 

 information in three dimensions but fail to show graphically the thickness of superposed 

 strata so they were supplemented by numerous north-south and east-west vertical sections. 

 These sections have been plotted on all of the coordinated lines shown on the maps but only 

 a few of them are here reproduced; all are available in GW-19. 



Contours are map projections of lines of equal elevation on the surface shown and contour 

 maps consist of contours spaced usually at equal vertical distances apart. A person following 

 a contour on the ground would travel continuously a perfectly level course. Contours also may 

 be described as the successive shorelines of the ocean if it were raised or lowered by equal 

 increments. From this it follows that a contour is always a closed figure, even though it has 

 to go around three continents for the purpose, and except for purely fanciful conditions a 

 contour can never be a loop on the end of a single line or cross itself. Contours swing inland 

 in a valley and out on a ridge. They are irregular on rough ground, smooth where the topog- 

 raphy is smooth. If close together, the slope is steep and vice-versa. A skilled topographer can 

 so draw contours as to give to an experienced observer a rather clear picture of the contoured 

 ground and not infrequently this picture will also reveal something of the underlying and 

 invisible strata and of the geologic history of that particular area. 



Contours shown on the accompanying maps lack precision. In making a contour map of 

 the ground, the topographer picks critical points of the landscape, summits, ridge lines, streams 

 and breaks in slope and determines the geographic location of these points and the surface 

 elevations at each. He then proceeds to sketch the contours by interpolation between these 

 points being guided at all times by the appearance of the terrain. It is axiomatic that for best 

 results contours should never be drawn by a man who has not seen the locality to be mapped 

 and that they should, if possible, be drawn in the field (as by a plane table survey) . 



Not only is this axiom violated perforce in this case because it is impossible to see the 

 surface, but also the points (wells) are usually poorly located for this purpose and the deter- 



151 



