se(|iient erosion, and experiencfd minor iijilifts. I'.lsc- 

 wliere in the central interior between the Appahichian 

 Mountains and the Great Plains, peneplanation was 

 the dominant force throughout the Mesozoic and Ter- 

 tiary. I-ow coastal marshes extended around the 

 Mississippi einbayment, and marshes and swamps 

 were frequent elsewhere. During the Pleistocene, 

 glaciers moved tremendous quantities of soil and 

 rock from Canada southward and from mountain 

 ridges into valleys. The retreat of the ice front pro- 

 ceeded haltingly with alternating retreats and ad- 

 vances. When it was stationary but melting, the 

 glacier formed concentric terminal moraines ; when 

 in active retreat, the glacier left a thick layer of till 

 in its wake. 



The Laramide orogeny, which occurred at the 

 end of the Cretaceous period, formed a series of 

 mountain ranges in the Rocky Mountain system, in- 

 cluding the Rig Horns. Wind River, Black Hills. 

 Uintas. and the series of more or less parallel ridges 

 in the Great Basin. Some of these mountains were 

 high enough to support glaciers during the Paleocene. 

 Rapid erosion filled the deep basins between the 

 mountains with debris. By the F.ocene. the moun- 

 tains were much reduced, and by the Oligocene pene- 

 planation was complete, although the surface was 

 several hundred meters above sea level and monad- 

 nocks remained. This peneplain extended to the Pa- 

 cific Ocean. 



Beginning in the Miocene, increasing in intensity 

 through the Pliocene into early Pleistocene, but de- 

 creasing since, mountain formation was extensive not 

 only in the Rocky Mountains but also in the Appa- 

 lachians, Ozarks and Ouachitas, Cascades. Sierra 

 Nevada, and Coast Ranges. \'olcanic action was ex- 

 tensive in the West, especially in Oregon and Wash- 

 ington. Highly fluid basalt welled out of long fissures 

 in the earth's crust, filled valleys, altered drainage 

 systems, and formed sheets up to 1500 m (5000 ft) 

 thick over 80,000 sq km (200,000 sq mi). Mount 

 Rainier, Mount Hood, and Lassen Peak were among 

 these volcanoes. During early Tertiary, the Sierra 

 Nevada and the Cascade Mountains were probably 

 only low ranges which were peneplaned by the Mio- 

 cene, but then both the Sierra and Cascades were up- 

 lifted by faulting and tilting so that the eastern edge 

 of the block was 4000 m (13.000 ft) above sea level. 



During early Tertiary, the area of the present 

 Coast Ranges was in part an island archipelago, sepa- 

 rated from the coast by a deep sea trough that is 

 now the interior of California. Folding and faulting 

 in the Coast Ranges began in the Miocene and were 

 most active in the Pleistocene. Such activity is still 

 going on as evidenced by the recent earthquakes. The 

 Coast Ranges are the youngest mountains in Xorth 

 America. 



CLIMATIC CHANGES 



The climate 60-70 million years ago can only 

 be deduced from sediment types and plant and animal 

 fossils. After peneplanation of the western mountains 

 there was little to obstruct the warm, moisture-laden, 

 westerly Pacific winds swee])ing across the conti- 

 nent. Rains were heavy, and fell fre(|uently through- 

 out the year. The Mississippi embayment helped 

 to maintain a uniform oceanic climate. Tropical 

 conditions extended as far north as the Dakotas and 

 Vermont, and temperate conditions obtained nearly 

 to tile North Pole. With the elevation of the western 

 mountains in the Miocene and Pliocene, especially the 

 Sierra Nevada and Cascades, the westerly winds were 

 forced to high elevations, cooled, and lost much of 

 their moisture as precipitation on the windward 

 western slopes during the winter ; dry seasons pre- 

 vailed during the summer. On the lee eastern moun- 

 tain slopes, arid conditions developed because the 

 winds, warmed as they descended the mountain, re- 

 tained what moisture remained in them, thus pro- 

 ducing a rain-shadoiv. Dry plains and desert thus 

 developed in the Great Basin. More moisture precip- 

 itated as the winds crossed the Rock-y Mountains. 

 Mixing of the westerly winds with winds from the 

 North and South, however, produced less aridity 

 east of the Rockies than in the Great Basin, and east 

 of the Great Plains the western mountain rain- 

 shadows had no efifect. 



Concurrent with increasing aridity over the con- 

 tinent was cooling of the air. This began in middle 

 or late Oligocene and brought a gradual southward 

 shift of climatic belts which culminated in the very 

 severe glaciation of the Pleistocene. The actual cause 

 of the glaciation is obscure, but there is no doubt that 

 the glaciation was accompanied by a drop in average 

 temperature, and an increase in annual precipitation 

 (Ewing and Donn 1956). 



EARLY TERTIARY FLORAS 



The geological record of Tertiary plants is 

 good, particularly in the western United States. Vol- 

 canic ash, lake deposits, coastal swamps, and river 

 basins preserved fossils well, more or less in situ. 

 These fossils indicate that there were three principal 

 floras, geofloras, or groups of plants that maintained 

 identity together over wide ranges of space and time 

 (Chaney 1947). It was from these floras and the 

 faunas that they contained, that the modern vegeta- 

 tion types, bionies, plant associations, and biociations 

 differentiated. Doubtless there was some latitudinal 

 and altitudinal zonation in the early floras, but the 

 development of present-day community units is the re- 



Paleo-ecology 281 



