84 PROCEEDIXGS OF THE AXX ARBOR MEETING 



Mr. C. K. Wentworth : While I am indebted to Professorl Hobbs for liis 

 suggestion as an addition to my collection of hypotheses for the origin of the 

 striated and faceted pebbles which are widespread in the basins of the James 

 and Potomac rivers, I am forced by the facts to take a very skeptical attitude 

 toward his suggestion. These pebbles and boulders were deposited in early 

 Pleistocene time. The evidence is overwhelming that they were striated in 

 earlj' Pleistocene. Great blocks and boulders, as well as blocks of fragile 

 rocks widespread over the coastal plain, point to ice flotation, indicating the 

 activity of ice far beyond that of the present time. The westernmost occur- 

 rence of the striated cobbles is at the edge of the Allegheny Plateau. 160 miles 

 west of any known areas of Triassic rocks. Professor Hobbs has suggested 

 the derivation of the striated pebbles of the Pleistocene from an area of Trias- 

 sic rocks which have since been removed bj' erosion. The convergence of 

 early Pleistocene grades with the present grades of the Potomac River at a 

 point near the westernmost occurrence of the striated pebbles indicates that 

 post-early Pleistocene erosion is here to be measured in scores rather than 

 hundreds of feet. The total removal since early Pleistocene time of a Triassic 

 remnant capable of yielding the large quantities of striated pebbles found on 

 the coastal plain is unlikely. So far as my familiarity with the Triassic con- 

 glomerates of the Potomac basin goes, I know of no pebbles or cobbles in these 

 rocks which in any important way resemble the smoothed, faceted, and striated 

 pebbles under discussion. The derivation suggested seems wholly untenable. 



Dr. SiDXEY Powers : I wish to call attention to the fact that the Squantom 

 tillite near Boston compares in age with the glacial beds in India. The Trias- 

 sic formations in Nova Scotia show no evidence of glaciation. 



Mr. George C. Martin : I wish to protest against the general acceptance of 

 faceted and striated pebbles as proof of ancient glaciation. Such evidence has 

 been generally accepted for the supposed continental Gondwana glaciation as 

 well as for glaciation during other periods in pre-Pleistocene time, and that 

 the faceted pebbles from the Newark beds afford such evidence appears to be 

 unquestioned by Professor Hobbs. The frequent present-day transportation 

 of glacial pebbles by ordinary river ice and by floating snags and driftwood 

 for long distances from their source in alpine glaciers or in eroded Pleistocene 

 deposits and their coming to rest in the fluviatile, lacustrine, or marine de- 

 posits that are now forming in non-glaciated regions is sufficient to show that 

 stronger evidence than even abundant glacial pebbles should be required as 

 proof of glaciation in the locality where the pebbles are found, or of a glacial 

 period at the time when the pebbles were deposited in their present position. 

 The presence of glacial pebbles in ancient conglomerates proves only, in my 

 opinion, that mountains have existed throughout nearly all geologic time, and 

 that the processes of precipitation, erosion, and transportation have always 

 been effective. 



Moreover, striated and faceted pebbles can be produced without the agency 

 of even local alpine glaciers. The banks of Yukon Kiver from high water at 

 least to low-water mark, in many places, have a natural pavement of cobbles. 

 The grinding of ordinai-y ri\er ice, with the embedded sand and gravel that it 

 picks up on the banks as it passes down the river each spring, has produced 

 facets, scratches, and polishe<l areas on the upper surfaces of the cobbles of 

 the pavement. That these marks are of non-glacial origin is shown by the fact 



