204 ILLINOIS STATE ACADEMY OF SCIENCE 



plenty of others which, if less well known, are quite as 

 beautiful and striking in their scenery, and quite as in- 

 teresting in their natural history. In all of them one gets 

 much the same kind of story : a great telescoping of suc- 

 cessional series and unusual groups of plants far out of 

 their ordinary range. Each of these places is a '^farthest 

 south" in its own region for such northern species as 

 white pine, yew, aspen, Canadian elderberry, harebell 

 and Arctic primrose. Each has outliers of southeastern 

 and southwestern floras. Naturally the eastern forms are 

 more numerous in Ohio and the western forms in Illinois 

 and Iowa, but the sigiiificant thing is that in any locality 

 the canj^on disjuncts represent a long jump from the 

 nearest open-country community of the same species. 

 Thus, a well-developed stand of sugar maple is a note- 

 w^orthy feature of the Iowa canyons, while at Sugar 

 Grove, Ohio, where sugar maple is a commonplace, the 

 rhododendron, -whose main range is in the southern AUe- 

 ghenies, becomes a sort of floristic equivalent. There is 

 no point in piling up examples; the character of these 

 canyons as botanical outjDOsts is evident at a glance even 

 to a casual tourist. 



The presence of these disjuncts, particularly of the 

 northern ones, becomes very suggestive when the location 

 of the canyons is examined in connection with the line 

 representing the farthest advance of the last (Wisconsin) 

 ice sheet. The accompanying map (fig. 1) show^s, in a 

 roughly approximate way, the relation of the half-dozen 

 canyons named in the preceding paragraph to the edge 

 of the Wisconsin drift and also to the driftless "island" 

 in southern Wisconsin and northern Illinois and Iowa. 

 The older drift exposures are omitted for the sake of sim- 

 plicity; what is shown is sufficient for the purposes of 

 illustration. Moreover, the erosion due to the release of 

 water from this, the latest of the glacial masses, is still 

 fresh, and the sides of the cliffs and canyons are still 

 actively weathering, so that the ecological factors here 

 are more active and more sharply contrasted than they 

 are in parts of the older drift not so immediately affected. 



It was on the sides and edges of these canyons that the 

 j&rst hardy tundra vegetation appeared when the last of 



