International Excursion in America. 
329 
the strange spectacle of the top of such a dune dotted over with 
perfectly healthy and vigorous examples of these trees and shrubs, 
representing the rejuvenated tops of individual members of a closed 
forest association buried many feet below the existing surface. 
None of these trees can, however, survive subsequent removal of 
the sand by wind. 
Erosion of Dunes. 
Besides the constructive phases of the sand dune succession 
outlined above, the international party visited an example (at 
Michigan City) of erosion of forested dunes by wave action. 
Here the advance of the lake, by undercutting the base of the 
sloping sand cliff, was causing the sand to slip down the slope 
from the eroding edge of the forest floor, sometimes slowly and 
gradually, sometimes in considerable masses. Here and there 
large masses of sand holding the roots of great forest trees had 
slipped half-way down the sloping cliff, and the trees were still alive 
and in the erect position. In other cases, of course, the trees had 
collapsed altogether, and lay prostrate upon or at the base of the 
cliff. Parallel cases of the erosion of clay cliffs with the gradual 
destruction of the forest occupying the top of the cliff were 
observed at Lake Bluff, north of Chicago. 
At this locality, too, a small sand-dune area with a somewhat 
different vegetation from that characterising the dunes on the 
southern shore of the lake was visited. Here the front of the 
dunes was occupied by a Calamovilfa association, closely resembling 
the Aiumophila associations of European maritime dunes. Behind 
this was a belt of Populus balsamifem, which replaces P. deltoides 
here and there on the southern dunes and increases towards the 
north, and then came a narrow but very well-defined belt of 
Junipenis horizontalis, a very characteristic shrub, with horizontally¬ 
growing branches, associated with Juniperus communis (with its 
typical form and also a form close to J. nana) and Arctostaphylos 
Uva-ursi, together with a few examples of Pinus Banksiana. At the 
back of the dunes, where these abutted upon the clay cliffs, the 
oak forest of the cliffs had sent a fringe of deciduous trees, shrubs 
and woodland plants on to the sand. The shore erosion of the 
lake cut obliquely across the face of this little dune area ; at the 
south end embryonic dunes were still forming, and following the 
shore northwards the erosion had reached successively older 
stages of the dune vegetation till finally it cut into the clay 
cliffs themselves, 
