Page:The Journal of geology (IA journalofgeology11893univ).pdf/275

some qualifications and exceptions. This conclusion is not at all new, for, as is well known, it has been reached by several students of these phenomena quite independently. But an approach to the question along the line of evidence presented by bowlder belts and bowlder trains has its own advantages. It bears particularly upon a new view of the origin of drumlins recently advanced by one of our most experienced glacialists in which they are held to have been chiefly formed from englacial drift which "had become superglacial by ablation and was afterwards enclosed as a stratum within the ice-sheet, being thence amassed in these hills."^[1]

In the midst of the drumlin area of south-central Wisconsin, there arise from beneath the Paleozoic strata a few scattered knobs of quartzite and quartz-porphyry from which erratics have been derived and borne away to varying distances, constituting bowlder trains of the most definite type. These are radically different from the bowlder belts discussed in my previous paper. The quartzite outcrops near Waterloo, in Jefferson County, are the most favorably situated for the purposes of the present study, because they are right in the heart of the most pronounced drumlin area and have made large contributions to the erratic content of the drumlins themselves. My associate, Mr. I. M. Buell, has been engaged for some time in a very careful study of the abrasion which these quartzite outcrops suffered from glaciation and of the distribution and special relationships of the erratic material derived from them. The drift movement was here to the south-southwestward and quartzite blocks derived from these outcrops enter in great abundance into the constitution of the drumlins lying in that direction. Several features of their distribution and nature are worthy of special note.

1. The quartzite bowlders are not simply scattered over the surface of the drumlins, but are distributed throughout their entire mass so far as accessible to observation. As the drumlins bear evidence of gradual accretion, it seems necessary to suppose that they were built up by successive additions of material