MAPPING. 23 



passing along the road, and after losing the whitish soil 

 of the chalk, we find, first sand, then a little farther on 

 gault clay ; no good sections, but still sufficient to enable 

 us to draw short lines where the road is probably crossed 

 by the boundaries. Once on the clay, we see nothing 

 else until we approach the stream, which runs through 

 flat marshy ground that appears to be quite level. This 

 flat marshy ground is " river alluvium," and as such 

 must be mapped a very easy matter, which consists 

 mainly in drawing a line where the slightly sloping 

 surface of the gault is lost in the level of the alluvium. 

 A contour line it is in fact, and necessarily so, seeing 

 that this alluvial deposit is the result of deposition from 

 the flood waters of the river which has occasionally 

 overflowed its banks. This line is best drawn in walk- 

 ing along it, or nearly so, down one side of the stream 

 and up the other ; it is found to run a little way up the 

 smaller streams and to die out just before we reach the 

 "W. margin of the slip. 



A clean section of the gault is seen in a pit by the 

 roadside, on the N. side of the river, but no other 

 evidence, although we walk over the flat at the N.W. 

 corner and down the W. side, until we strike the foot- 

 path when gault is again visible. Following the foot- 

 path, at the rising ground we come upon sand, note 

 approximately the junction, and a little higher up find 

 by digging that the sand passes in under the chalk. It 

 is now a matter simply of continuing to draw these two 

 boundaries, the lower one passes through a spring, by 

 which the general accuracy of the work is proved, and 

 both cross the road where we had drawn the short pro- 

 visional lines soon after starting. 



