154 THE BOOK OF NATURE STUDY 



the lower contours being closer together (Fig. 47). Cuestas or wolds 

 are hills which have one side the scarp or escarpment side steep, 

 and the other the dip-slope falling much more gently (Figs. 44, 

 45, 48). On the scarp side the contours run exceptionally straight 

 and parallel to the summit-ridge, which often extends for many 

 miles, dividing the steep side from the other. If valleys cut across 

 them they often widen and narrow alternately, the contours crowd- 

 ing together where the wolds are crossed and spacing out in the 

 flatter ground between (Fig. 48). Isolated hills, sometimes called 

 nabs (or buttes) , are surrounded by closed contour curves, and gener- 

 ally have valleys radiating out from them (Fig. 52). Hogs' backs, or 

 ridges gently sloping from end to end, but with abrupt sides, are 

 indicated by contours of a much elongated oval shape (Fig. 47). 

 Rocky ridges or ar&tes, have elongated contours often with sharp 

 angles, which might at first be mistaken for valley Vs. Most of 

 these characters can be verified in the contour maps (Figs. 44-48). 



Tracing a valley up to its head, the contour above the highest 

 V will approach its fellow on the other side of the valley, and then 

 recede from it again. This marks a col or pass, the depression 

 across the high ground joining the heads of two valleys, the water 

 draining in opposite directions from its summit. It is along such 

 cols that roads will be seen passing through hill ranges from one 

 valley or low ground to another. Roads traversing high land, 

 on the other hand, cross cols at right angles to the valley roads, in 

 order to avoid as much as possible dipping down off the ridge lines 

 (Fig. 48). 



Watersheds and drainage areas may be traced out with exactness 

 on a contour map, and they should be indicated by coloured lines, 

 principal and subsidiary watersheds being marked by difference in 

 width of line. Watersheds are generally situated on high ground 

 along ridges, scarps, plateaux, and they cross the cols (Figs. 47 and 

 48). But it does not necessarily follow that the loftiest and most im- 

 portant hills are principal watersheds, for rivers may rise in ground 

 of no great average height, and may in their course traverse and 

 cut deep valleys in considerable hill ranges, of a height now greater 

 than the summit of the watershed. Valleys of this description 

 are known as transverse valleys, and they are often in parts of 

 their course steep sided and gorge like. The valleys tributary 



