Page:The Journal of geology (IA journalofgeology11893univ).pdf/147

sections are reported to have passed through a black mucky clay, probably a preglacial soil, immediately beneath the blue till and a few feet above the rock surface. This feeble abrasion is thought to be due to lack of vigor in the ice-movement. The attenuated border is apparently due to the same lack of vigor and to a comparatively short occupancy of the region by the ice-sheet. The lack of vigor in this earlier invasion is in striking contrast with the vigor of the invasion which produced the outer moraine of the Miami and Scioto lobes, there being numerous exposures of striation in the district immediately north of that moraine, while the moraine itself bears evidence that the ice-sheet had great shoving power.

There is a possibility that this earlier drift sheet embraces two distinct periods of deposition. Evidence in support of this view is cited by Professor Orton in his report on Clermont county, Ohio (the county bordering the Ohio river just above Cincinnati), viz., that a buried soil and deposit of bog iron occur at the junction between the yellow and blue tills. From Professor Orton's account it would appear that no marked oxidation of the surface of the underlying blue till had occurred before the yellow till was deposited. We infer from this that the interval of deglaciation may have been comparatively brief, though it is possible that swampy conditions, such as prevail in the production of bog iron, prevented oxidation during a prolonged period. Inasmuch as Professor Orton is a careful observer and cautious writer, I do not feel free to question the evidence he cites, but my examinations in this district have not confirmed his evidence, so far as the location of the soil bed at this particular horizon is concerned. I have found testimony as to the occurrence of buried wood at or near this horizon, but not of soil beds. Possibly Professor Orton considers the occurrence of wood good evidence of an old land surface, but in view of the fact that wood may be incorporated in the drift as a part of the glacial debris I have not thought this a sure evidence. In the lists given in the Ohio reports, Dr. Newberry cites instances of wood to prove the existence of a "forest bed," and forest