128 
Psyche 
[December 
Field Survey 
The Palos Park region lies in the Valparaiso moraine 
about twenty miles southwest of Chicago. It is a rolling 
upland, situated to the south and east of the Des Plaines 
river, traversed by the Sag 1 , a small stream flowing in a 
westerly direction into the Des Plaines. Most of the region 
is covered with a thick growth of the mesic oak-hickory 
type of subclimax forest characteristic of the morainic up- 
lands of the Chicago region. (Cowles 1901). The mound- 
nests of Formica ulkei found here are concentrated in an 
area of approximately one quarter mile square in which 
the forest is cut up by a number of open lanes of swampy 
meadow providing for local drainage towards the west. 
Several temporary ponds are present in the spring but by 
early summer there is no surface water except at times of 
rain. The substratum consists of a thick layer of glacial 
drift, yellow clay and boulders, with a thin layer, two to 
six inches, of black humus surface soil. 
A careful survey of this area in which all the mounds 
were tabulated according to size, location, and activity, re- 
veals the presence of 435 mounds of Formica ulkei. These 
vary in size from mounds about one foot in diameter and 
six inches in height to five feet in diameter and three feet 
in height. Typically, the mounds have a somewhat matted 
thin surface layer composed of finely divided soil and or- 
ganic debris such as twigs, stems, bits of leaves, and frag- 
ments of insects. The surface is perforated with many en- 
trances to the interior which is finely ramified with well 
defined galleries, one to two centimeters in diameter. These 
extend throughout the mound and into the substratum down 
to the soil-water level from one to five or six feet below 
the surface. (Holmquist 1928). In most cases the mounds 
are free from living vegetation. The smaller mounds are 
round to oval at the base and bluntly conical in shape. 
The larger mounds are in general somewhat elongated and 
consequently show a definite orientation in that the long 
axis is east and west. In addition, the south face has a 
longer more gentle slope with considerably greater surface 
1 At the present time the Sag canal is used for drainage. 
