486 
Buell—Bowlder Trains . 
From the summary of the last article the following facts are 
gleaned which will serve to bring the general geological rela¬ 
tions into brief review. 
The ledges all lie in the basin of Crawfish river, a small trib¬ 
utary of the Rock in southeastern Wisconsin which is crossed 
about midway by the railway line joining Madison and Water- 
town. The area is marked by profound preglacial erosion, the 
region being formerly, no doubt, within the horizon of the up¬ 
per Silurian, but now these strata are entirely removed, the 
quartzite ledges appearing in the horizon of the St. Peter’s 
sandstone and the adjacent marsh basins being cut quite 
through the lower Silurian, exposing the Cambrian strata. In 
the ridge crests adjacent to the valley on the west the lower 
beds of Trenton limestone appear. 
The accompanying map, plate XII, gives the topographic feat¬ 
ures of that part of the basin which contains the outcrops, in¬ 
dicates the relations of the outcrops to one another and to the 
forms of surface relief, and shows the surface distribution of 
the quartzite drift upon the proximate areas of their boulder 
trains. 
The outcrops, numbering forty or more, counting brief ex¬ 
posures, are grouped into four clusters which are separated by 
considerable intervals of drift-covered surface, which is under¬ 
laid by the lower Silurian rock of the region. The areas have 
been designated from their geographic relations the Portland 
outcrops, the Hubbellton group, the Mud Lake ledges and the 
Lake Mills ledge. From the correlation of observations upon 
the dip and strike of the strata exposed in these ledges, it is 
evident that they represent the remnants of one or more pro¬ 
found synclinal folds, which appear to have a direction between 
N. E.-S. W. and E.-W. The Portland and Mud Lake out¬ 
crops belong to the northwest margin of a fold, and the sharp 
turn in the strike shown in the southern exposures of the Port¬ 
land area indicates an easterly pitch of the same fold which 
comes to the surface in that vicinity. The more gentle dip of 
the strata in the Hubbellton area marks a position near the 
bottom of the trough, and the sharp northwest dip in the Lake 
Mills outcrop may represent the reverse side of another fold. 
