BY E. C. ANDREWS. 



805 



fallen into disrepair. The peculiar contours of these gutters are 

 well known to be developed during severe rainstorms. The 

 forms depend upon the material acted upon; this may be homo- 

 geneous and coherent; it may be soft basally and have a hard 

 capping; or it may be soft above and hard below. The homo- 

 geneous and coherent structures alone will be here considered, as 

 the reader may easily reason out the contours for the non- 

 homogeneous structures from a knowledge of the forms produced 

 in the simplest case. A disused foot, bridle or wheel-track in 

 tenacious clay or rotten rock very frequentl}'' constitutes the 



Fig. 3. — Trench with amphitheatrical (or cirque-like) head 

 formed in hard clay during heavy thunderstorm. The trench 

 originated in a basin, as shown in figs. 1 and 2; and the recession 

 of the basin-head caused the long gutter with its straight sides and 

 broad base. The figure illustrates a possible origin of fiord-basins. 



original valley. These tracks may have been but a few inches 

 deep and less than a foot in width. In regions subject to violent 

 thunderstorms the valley, after several years, is probably from 

 one to two feet in depth and has, comparatively, a very wide 

 floor, straight or very steep sides, spurless walls, a fairly uniform 

 grade broken every here and there by amphitheatrically-hea.ded 

 trenches, the amphitheatre bases existing in the form of basins, 

 deepest near their upstream ends. Stream debris is commonly 

 plentiful near the downstream end of the basins. Again, basins 



