Origin of Vie Dells of the Wisconsin. 557 
of the creeks and branches alike may head in a col into 
which the water falls from the comparatively level land above 
in times of melting snow or abundant rainfall. In the dry sea¬ 
son in many of the branches no water falls from the plain above. 
Such a dell thus starts almost at once as a canyon from 15 to 50 
feet deep. The head of the col is found to be located upon 
a joint, which widened by erosion at the head of the col 
may be traced back to a mere seam. This slit ahead of the 
col and in the direction of the stream plainly controls the 
backward cutting. Other dells end by opening out and rising 
with a gradual or steep slope to the plain above but these have 
a set of right angles turns the same as these which head in a col. 
As the streams become of larger size in the lower reaches, that 
there is a rectangular turn in passing from one set of joints to 
the other may be obscured by the fact that the corner has been 
worn off, thus making a curve, camparable to a curved joint 
in a stove pipe when it takes a new direction, rather than the 
rectangular joint which used to be so common, and which the 
stream undoubtedly had at first. However, in the newer dells 
the rectangular system is almost perfectly preserved, and at 
several places along each dell one may see the beginning of new 
side streams, which have opened up a joint a few feet to many 
feet from the main dell. The different dells vary in length from 
those which have just begun to follow the joint back from the 
Wisconsin to those which show a half dozen or more right angle 
turns. The older any given stream, the larger the area of the 
plain above which has become tributary to it. In some cases 
the dells follow joints other than those of the rectangular 
system. 
Because the dells are located on joints, erosion is easy down¬ 
ward and gorges are cut very quickly to the level of the main 
stream. This easy vertical cutting as compared with the side 
cutting is the cause of the picturesque features of the dells, 
which are perhaps fifteen to fifty feet deep, and in places 
barely wide enough for a person to walk through. The down¬ 
ward cutting soon results in a base level, and makes the chief 
work of erosion the corrasion of the sides of the canyon. A 
