Page:The Journal of geology (IA journalofgeology11893univ).pdf/277
hypothesis that the erratics rose in the ice and were transported englacially in any considerable degree. In this case, the greatest accumulation should have been at the terminal moraine where the ice halted longest. It seems to be also difficult to understand, on the hypothesis offered by Mr. Upham, how the drumlins lying immediately on the ledges and immediately in their lee could have been filled so largely with quartzite-bowlders. Certainly the bowlders could not have risen in the ice so high as to have become exposed at its surface by ablation and then have been overflowed by a new accession of ice and moulded into the drumlin form, and at length have been let down by the melting of the ice beneath without more forward movement than observation shows. The simplicity of the facts do not seem to tally with the complexity of this theory.
- The amount of abrasion which the bowlders suffered bears specifically upon the question of the mode of their transportation. The parent outcrops gave rise to erratics of three kinds. (1) In Paleozoic times the ledges stood as islands in the seas, and there accumulated about them very coarse conglomerates of quartzite. From these, a portion of the erratics were derived in an already rounded condition. The character of this rounding and the superficial changes the pebbles underwent before the glacial period made it possible for Mr. Buell to distinguish these with measureable certainty in following the train until abrasion had destroyed their surface characters. (2) Talus blocks accumulated about the bases of outcrops before the ice invasion that formed the drumlins. There was an earlier invasion which bore quartzite erratics westward. The later invasion bore them southwestward. The talus blocks under consideration are, perhaps, in the main, those that were derived from the quartzite knobs in the interval between the two. They are distinguishable from blocks disrupted by the ice by means of the weathered character of their several surfaces, so long as these remain unabraded. (3) The third class of erratics are those that were derived by direct action of the ice upon the parent knobs. These distinguished by their unweathered fracture surfaces.