June, 1931 



EVOLUTION 



Page thirteen 



while the other end is often abrupt and much broken because 

 the ice froze to and plucked away pieces of the rock mass. 



This work of planing and carving the underlying hard rocks 

 could hardly have been done by the ice itself, it being too 

 yielding. But the hard rocks frozen into its bottom could 

 well serve as hob nails for the great heel of the ice. They 

 could plane, and scratch, and even groove the bedrocks. A 

 notable example of such grooving is found in the big slab from 

 a Lake Erie island which is displayed at the entrance to the 

 American Museum of Natural History. Lesser grooves are 

 to be found in many places throughout the city, and here and 

 there the once numerous glacial scratches. Originally, quite 

 'all the surface bedrocks were probably full of such scratches, 

 but long exposures to weathering have destroyed them. Only 

 where the top soil has been recently removed, do we find clear 



Terminal moraine and directions of ice movement in New York 

 vicinity during last glaciation. 



glacial scratches. In the Bronx Botanical Gardens are several 

 fine examples. In one place, the scratches cross each other at 

 an angle, indicating that the ice movement changed direction. 

 It is even possible to determine which were first made and 

 which later. The Botanical Garden also furnishes examples 

 of quite all the other signs of glacial action, even to a small 

 glacial valley, really a big glacial groove, gouged out by the 

 ice stream that forced its way through. Such a valley, usually 

 much larger, is Ul-shaped in cross-section, for the icy mass 

 tends to spread and fill it fully, cutting along the sides as well 

 as center. A stream-cut valley is instead V-shaped, the water 

 working only in the narrow central channel. 



Then there are glacial pot-holes, some near the best ex- 

 posure of scratched bedrock, others along the gorge of the 

 Bronx River. They were undoubtedly caused by the eddying 

 waters of falls or rapids, which used rock fragments and 

 ^ gravels as grinding tools, swirling them round and round 

 until holes were bored several feet deep and wide in the hard 

 bedrocks. These, and some channels over the rocks that are 

 plainly stream-worm, are very strangely placed, high up where, 

 it would seem, no stream could possibly flow. But the great 

 glacier helps us explain, for in it and under it were confining 

 ice tunnels in which flowed swift, gravel laden streams, milling 

 violently round each rock obstruction, with now and then a 



waterfall plunging down some icy crevasse. The deepest of 

 these pot-holes is very obviously due to a waterfall; the next 

 largest has a big tree growing in it. 



These two are near the upper end of the Bronx Gorge. 

 This gorge is obviously young, speaking geologically, for it is 

 deep and narrow, quite unlike the valley upstream, which has 

 had time to widen by long continued weathering and lateral 

 erosion. Judging by the rate at which streams elsewhere are 

 known to cut into such rocks as these, this gorge is only about 

 35,000 years old, which brings us back to the estimated date 

 for the close of the Ice Age. This, and the fact that we find 

 an old, abandoned stream valley to the west, which the rail- 

 road now follows, leads us to conclude that the ice sheet must 

 have dumped so much rubbish into the old channel as to dam 

 and divert the Bronx River to its present course, forcing it to 

 cut its new gorge through the hard rocks. This conclusion is 

 confirmed farther upstream when we come to a little roimd 

 lake at an old bend Ln the original river charmel. 



More extensive and detailed studies have convinced the gla- 

 cial geologists that, at the close of the Ice Age, a series of big 

 lakes formed before the retreating ice front, covering much 

 of the country to the south and east of Bronx Park, and filling 

 other valleys to the west. One of these. Glacial Lake Passaic, 

 was formed between the Watchung mountains and the ice 

 front. With the retreat of the ice it was completely drained, 

 but we can trace its old shore lines by the beaches at its former 

 high levels, and the present clay beds represent its former bot- 

 tom deposits. 



To the eyes that see, the Bronx Botanical Garden tells 

 another absorbing story of plant adaptations and migrations, 

 exemplified by the fine stand of Northern Hemlocks above 

 the gorge. These trees are native to Canada and a cooler cli- 

 mate, but when the cold, hard glacier crept down, these tall, 

 proud trees marched south in wise retreat, migrating slowly 

 by generation after generation throwing out their seeds in ad- 

 vance. Then when the hot sun chased the glacier out, they 

 marched back home again but left this outpost grove of those 

 that were lured to stay on in this garden spot. 



BROMS ON THE AIR 



r7ACH Saturday evening at nine o'clock Allan Broms broad- 

 ^—^ casts popular science over station WOR, each subject, 

 though complete in itself, being also preparation for a field trip 

 or museum visit on the Sunday after. The June program is as 

 follows: — 

 June 6, WOR, 9 P.M. THE WAYS OF THE WILD. 



June 7, 10:30 A.M. Meet at Bronx Park subway entrance 



to N. Y. Zoological Park. 

 June 13, WOR, 9 P.M. THE TALE OF THE HORSE. 



June 14, 1 P.M. Entrance American Museum of Natural 



History. 

 June 20, WOR, 9 P.M. THE IRON HORSE. 



June 21, 2 P.M. Lobby Museum of Science and Industry, 



220 E. 42nd St., New York, N. Y. 

 June 27, WOR, 9 P.M. RESTORING ANCIENT MAN. 



June 28, 1 P.M. Entrance American Museum of Natural 



History. 



The Saturday Radio talks by Mr. Broms will continue over 

 WOR. Details of the correlated field trips are announced dur- 

 ing each broadcast. Mr. Broms also broadcasts Saturdays at 

 7:15 P.M. over station WPAP. 



