IN THE END OF THE OLIGOCENE INFERIORIS A GREAT CLIMATIC CHANGE APPEAR (AND NOT A PEAR IN PARADISE) THIS CLIMATIC CHANGES ARE BRUTAL AND ENDURE 4 MILLION YEARS THE LAST ONE IN THE BEGGINING OF THE OLIGOCENE ONLY LAST SOME HUNDREDS OF THOUSAND YEARS BIG DROP IN THE OCEANIC LEVEL...THE AVERAGE PRECIPITATION DROPS FROM 500 TO 900 mm A YEAR TO 350 TO 450 mm OR IN INCHE'S DIVIDE FOR YOU KNOW...OLIGOCENE SEDIMENTATION, STRATIGRAPHY, PALEOECOLOGY AND PALEOCLIMATOLOGY IN THE BIG BADLANDS OF SOUTH DAKOTA JOHN CLARK Associate Curator, Sedimentary Petrology, Field Museum JAMES R. BEERBOWER Department of Geology, McMaster University, Hamilton, Ontario KENNETH K. KIETZKE FIELDIANA: GEOLOGY MEMOIRS Climate A fairly precise estimate of average temperature immediately preceeding Chadron time can be made. Since lateritic weathering of rocks other than limestone requires an average annual temperate of 60° F or higher (Krynine, 1949), the slight to moderate lateritiza- tion of the underlying Pierre shale and the slight lateriti- zation of some Chadron sediments suggests a tempera- ture of 60° F or very little higher. A mean annual tem- perature of 60-65° corresponds with the present situa- tion in southeastern United States. Comparison of the pre-Chadron weathering in South Dakota with that in Montana throws light on the vertical temperature gradient. Near Whitehall, Mon- tana, at present elevations of 4000-4500 ft., the Paleo- zoic limestones show pre-Chadronian lateritization, but all other rock types were weathered to limonitic or kaolinitic debris. This indicates (fide Krynine) a mean annual temperature of about 55-60°, like that of West Virginia or of Cairo, Illinois. Assuming the average an- nual temperature in Montana to have been 57-58°, and the difference in elevation between the two stations to have been the same as at present, 1300 ft., the vertical temperature gradient in pre-Chadronian time would be about 3° per 1000 ft., or the same as the present gradi- ent. Since the adiabatic gradient depends upon un- changing laws of physics, the accordance of the esti- mated temperatures to the gradient supports the ac- curacy of the estimates. The presence of small alligators in the Ahearn and Crazy Johnson Members can be taken to mean that winter minimum temperatures did not long remain be- low freezing during much of Chadron time. It does not, however, indicate that the average annual temperature was as high during Chadron time as it had been before. The very incomplete lateritization of Peanut Peak sediments in the neighborhood of the major Chadron stream courses, and the lack of oxidation in Chadron sediments away from those stream courses, indicates that temperatures were not as high during Chadronian time as they had been before. Furthermore, the decrease in amount of weathered upland debris deposited in Peanut Peak sediments as contrasted with that in the upper Ahearn suggests that weathering processes in the uplands progressively declined through Chadronian time. This would suggest a drop in temperature, or precipitation, or both. Dorf (1959, pp. 185-189) cites paleobotanic evidence indicating that the later Oligo- cene was cooler than late Eocene. An estimate of annual precipitation is much more difficult. Lateritization is accomplished under condi- tions of abundant but highly seasonal precipitation — 40 in. or more, with an alternation of rainy and dry sea- sons. It may be safe to assume a pre-Chadronian annual rainfall of over 40 in. The Chadron sediments themselves include algal limestones, casts of Unio and pond snail shells, bentoni- tized ash, and predominately reduced disseminated iron (greenish to bluish color), all of which indicate abundant water. They also contain scattered zones of gypsum crystals, manganese dioxide, and, especially in the Peanut Peak member, light tan sediments and cal- cereous nodules or zones, which suggest aridity. The fauna is a mixture of wet-forest forms such as alligators, small artiodactyls, insectivores, and Mesohippus with such dry-plains animals as the camel, Poebrotherium. This apparently conflicting evidence resolves itself into a co-ordinated picture when it is noted that the indications of moisture occur generally near the bottom of the section or concentrated in the vicinity of the chan- nel fills. The evidences of aridity, on the other hand, are to be found away from the channel fills and near the top of the section. Abundant run-off from the Black Hills, with the streams passing through a relatively dry plains area, could produce this pattern of evidence, especially if desiccation became progressively more severe. INTERPRETATION OF CHADRONIAN SEDIMENTATION General Review of Tertiary Sedimentation A. Description. The Oligocene epoch was a time of transition — this concept has been well documented by faunal studies, but the evidence to be derived from study of major lithologic changes has not been systematically pre- sented. Figures 26 and 27 review the senior author's observations on Tertiary continental deposits and in- clude distribution, thickness, lithology, and evidence of depositional environments of most of these sediments from the High Plains west to the Nevada-Utah bound- ary and from the San Juan basin north to the Canadian boundary. Much of the data on which this chart is based can be verified in the literature. However, we have omitted data which could be gleaned from the literature but which have not been personally observed because it seems unfair to quote in this context descriptions made without anticipation of this interpretation. The chart shows that Upper Paleocene and Wasa- tchian sediments are primarily fluvial, of wide distribu- tion and moderate thickness. Sediment colors are gen- erally dark reds, greens, and purples, and the fossil bones are heavily impregnated with iron and manganese oxides. Middle and upper Eocene deposits on the other hand occur in only a few of the intermontane basins, are usually thicker than the underlying Tertiaries, and contain a large proportion of lacustrine sediments. Middle Eocene sediments are generally pale greenish to grey or tan, with bones colored pale tan by limonitic im- pregnation or black with manganese dioxide. The color of upper Eocene rocks and impregnation of the fossils is generally similar to that of the Wasatchian. Deep pre- CLARK AND BEERBOWER: THE CHADRON FORMATION 61 Chadronian weathering in South Dakota, Nebraska, eastern Wyoming, and Montana suggests that these areas were exposed surfaces during late Eocene time and that there were no late Eocene sediments deposited in this region. In contrast, Chadronian sediments occur in north- eastern Colorado, western Nebraska and the Dakotas, many of the basins of Wyoming, and most of the inter- montane valleys of Montana. They rest discomformably on eroded, deeply weathered older rocks — only at two places, Beaver Divide and Sage Creek, were they de- posited conformably on late Eocene beds. Bentonitized ash is always present in the Chadronian sediments and increases in amount and freshness toward the top of the section. The Chadronian sediments are thin, commonly less than 200 ft. thick, and occur only on the eastern side of the Continental Divide. The sediments are primarily fluvial and have relatively pale colors. Fossil bone is slightly impregnated, but sometimes heavily coated, with limonite. Post-Chadron deposition in general parallels that of the Chadron in distribution and characteristics. The color of Orellan rocks is somewhat paler and more tan than Chadronian; the ash content is relatively higher; and fossil bone occasionally has a coating of hematite but otherwise shows little more iron-manganese im- pregnation than modern bone found on the surface of the High Plains. In Figure 26 these changes from the Paleocene through the later Tertiary are summarized and the transitional nature of Chadronian deposits is demonstrated. The development of reddish tan sedi- ments, immediately followed by non-deposition, at the close of Chadron time, should be noted. B. Interpretation The thickness and distribution of the deposits must reflect both tectonic and climatic controls. The propor- tion of lacustrine to fluvial beds must be the result primarily of tectonic activities by which the basins were blockaded to form lakes. On the other hand, sediment color if syngenetic, and bone impregnation, should be functions of climate rather than of tectonics. Sediments may, of course, inherit their color from the parent rock or weathering of that rock, or may de- velop it epigenetically. The senior author has de- termined certain field criteria for the recognition of derived, epigenetic and syngenetic colors (Clark, 1962), and we limit our discussion to the latter, since they alone are significant in climatologic interpretations. Highly colored fluvial sediments are generally deposited under conditions of high temperature and humidity with abundant vegetation; pale fluvial sediments indi- cate aridity but not necessarily cool temperatures. The nature of impregnation of fossil bone is also partially controlled by local climate. The senior author has found that in most semi-tropical to tropical forest environments fossil bone is heavily impregnated with hematite, limonite, (in general, hydrous iron oxides) and manganese oxides. In moist and somewhat cooler en- vironments there is an impregnation of brown limonite. Several horse skeletons buried for 35 years in forest mould near Princeton, New Jersey, showed such im- pregnation to depths of one-eighth inch. Burial under somewhat drier conditions seems to produce incrusta- tion without impregnation; for example, a woodchuck skull recovered from swamp mould in northeastern Illi- nois was heavily encrusted. In contrast, burial on the semi-arid high plains in Dakota and Colorado produces no iron impregnation. The precise factors that control impregnation are not known but are probably related to the acidity of the local ground water, and will vary with the nature of entombing sediments, porosity of the bone, and speed of burial. However, the generalization as to climatic conditions fits all the available evidence. the thin, widespread deposits across the central High Plains. These are: 1. eustatic rise of sea level; 2. uplift of a structural barrier across the lower reaches of the depositing streams; 3. down-warp of the entire region; 4. overloading of streams with volcanic ash; and 5. climatic change sufficient to cause a shift in stream regimen. Eustatic rise of sea level can be eliminated as a pos- sible cause for several reasons. First, the coastal Plain stratigraphy gives no evidence in favor of it, and sug- gests rather that Oligocene sea level was lower relative to the land than was Eocene sea level. Second, the Atlantic drainages south of central Colorado did not begin deposition simultaneously with those to the north, as they should have done had there been a change in sea level. Third, the Chadron of South Dakota thins both mountainward and seaward from its maximum thick- ness about 20 miles east of the rim of the Black Hills. A rise in sea level might be expected to result in a deposit thinning progressively upstream from the river mouth. Uplifting of a structural barrier across the lower reaches of the depositing streams would also produce a lens of sediment thickest near the downstream shore of the resulting lake and thinning upstream. Further, the Oligocene streams of southwestern Montana probably did not enter the same master stream as did those of central Colorado. Therefore, a tremendously long struc- ture, no trace of which is known, must be called into being. This second possibility can also be discarded. A gradual downwarp of the entire area from Idaho to the Break of the Plains and from the Canadian border to central Colorado, has been postulated in oral discus- sions as the cause of mid-Tertiary (Oligocene-Miocene) deposition. Three main points of evidence have been regarded as supporting this hypothesis: 1. Presence of Oligocene and Miocene outliers at elevations up to 7500 (Montana) and 9000 ft. (Big Horn Mts.). If these levels are projected out into the inter- montane basins, the late Miocene bottoms of the basins as reconstructed would lie 3500-4000 ft. higher than the present ones. Such great masses of fill, in every basin and trough, could only be explained by filling of a down- warped area. 2. The small amount of erosion of mountain slopes since early Oligocene time. Basal Chadronian sediments everywhere contain pebbles derived from rocks exposed in the present cores of the adjacent ranges. Chadronian sediments choke the mouths of present canyons de- bouching from the various mountain ranges, and thus indicate that early Chadronian mountain topography must have been like the present topography. This pres- ervation of an ancient surface can be explained by as- suming burial under thick mid-Tertiary sediments, which were then removed during Quaternary time. 3. Widespread superposition of streams on low ranges or near the ends of long ranges can be explained by assuming a cover of Tertiary sediments on which the streams meandered before their recent incision. These are strong evidence, but other reasoning, equally cogent, opposes them: 1. There is no direct structural evidence of any such downwarping. 2. There are no known angular unconformities within the mid-Tertiary section (except in the Slim Buttes, where the movement is either non-tectonic or extremely local). If such extensive downwarping oc- curred, differential movements causing angular uncon- formities would be expected. 3. Basal Chadron sediments are everywhere (except at Beaver Divide) composed of reworked, deeply weath- ered material, but later elastics usually consist of fairly fresh rock, showing that once the pre-Chadron soil had been stripped, weathering did not keep pace with ero- sion. An area undergoing downwarp would certainly not be expected to show accelerated erosion. 4. Depositional dips of 3-5° are usual in the montane mid-Tertiary sediments, and demonstrably initial dips of up to 25° have been noted. Deposition of uniform, thin strata on a porous surface at even greater dips can be demonstrated experimentally. If average depositional dips of 2° are presumed, which is conservative judging from the observed dips, then the mid-Tertiary fill of the Big Horn, Powder River, and other major basins need not have been more than 1500 ft. thick. This thickness accords roughly with the thickest preserved sections, and, further, the deposi- tional structure would accord with that of the observ- able sediments. It is our belief that, with the exception of a few locally downfaulted areas in Montana, none of the basins of this area ever contained more than 1500 ft. of mid-Tertiary sediment. 5. The cases of stream superposition do not demon- strate a fill deeper than 1500 ft. Several streams are superposed over very low ranges, with much higher mountains nearby. Others traverse the lower reaches of higher ranges. In no case is the top of the ridge at the watergap more than 1500 ft. higher than the recon- structed pre-Oligocene surface. 6. Since pre-Chadron topography was probably very much like that of the Recent, the major stream systems would have filled rapidly to grade with clastic materials from the mountains during downwarp. Yet the post-Chadronian sediments contain smaller clastic constituents with the major proportion consisting of ash. Therefore, post-Chadronian deposition may have been partially controlled by the supply of volcanic ash and might be expected to show a different distribution than the Chadronian deposits. 7. Downwarp might be expected to cause Chad- ronian deposition on both sides of the Continental Di- vide rather than on one side. 8. Progressive downwarp should cause the youngest sediments to be thickest near the center of downwarp but these later Oligocene and Miocene sediments are CLARK AND BEERBOWER: THE CHADRON FORMATION

thickest near the base of the local mountain range that 
formed the watershed of the streams depositing each 
series. The only regional trend in thickness appears to 
be related to an increase in volume of volcanic ash 
toward northwestern Wyoming. 

These eight lines of evidence weigh against the 
probability of a regional downwarp as the basic cause of 
deposition. 

The fourth hypothesis, the overloading of the 
streams by volcanic ash, has two points in its favor. 
First, the ash occurs in large volume throughout the 
upper part of the section and increases in amount, pro- 
portion, and average grain size toward the volcanic 
center. Second, Chadron deposits are absent west of the 
volcanic center, and south beyond the zone where pre- 
vailing westerlies might be expected to carry ash. 

Several strong lines of evidence militate against ash 
as a primary cause of mid-Tertiary deposition. First, 
the basal 20 ft. of Chadron sediment nowhere include 
volcanics. In South Dakota, ash first becomes an im- 
portant part of the sedimentary mass in the Crazy 
Johnson member, and does not become dominant below 
the Peanut Peak Member. Overloading with ash cannot 
reasonably be regarded as the primary cause of an epi- 
sode of sedimentation which began without ash and did 
not receive significant quantities of ash until the old 
topography had been buried. Therefore, increase in 
volume of ash in the streams could not have been the 
initial or the primary cause of deposition, but it prob- 
ably influenced the rate of deposition. 

The remaining hypothesis, that of climatic control 
of stream regimen, is thus supported by the default of 
the other four suggested; furthermore, the positive evi- 
dence for this interpretation is substantial. 

The occurrence of a major climatic change at the 
beginning of Oligocene time has been demonstrated 
adequately in this and other papers. The change from a 
moist, warm, and possibly monsoonal climate to a semi- 
arid, cool climate would profoundly alter vegetational 
cover, weathering, and stream discharge and conse- 
quently would modify stream regimen. 

In late Eocene time weathering must have been 
primarily chemical and resulted in a predominance of 
clays and solutes. The heavy vegetational cover would 
restrict surface run-off and thus reduce the amount of 
sediment relative to stream discharge. The master 
streams — adjusted to this comparatively small sediment 
load — would have low gradients over all outcrops sus- 
ceptible to chemical weathering. 

On the other hand, the rocks resistant to chemical 
weathering in the mountain cores would rise abruptly as 
sharp ridges, and consequently stream profiles would 
change rather abruptly near the divides. (Cotton, 1941 
p. 155-156; Davis, 1923 p. 21; Lawson, A. C, 1932 p. 
706). The landscape would then comprise two sets of 
features; broad river valleys with gentle gradients de- 
veloped on the shales and weakly cemented sandstones, 
and bold mountain ranges on the more resistant rocks. 



In Chadron and post-Chadron time the cooler, drier 
climate must have produced a relative increase in 
mechanical weathering with a resultant increase in 
supply of coarser elastics. The vegetational cover must 
also have been reduced, and surface run-off consequent- 
ly increased in relation to total run-off. In turn, in- 
creased surface run-off would increase slope wash and 
gullying on steeper slopes. The effective load of main 
streams would therefore be relatively large at the same 
time total water discharge was decreasing considerably. 
The net result would be erosion of upland slopes, remov- 
ing weathered mantle first and then relatively fresh 
rock fragments, and deposition in the major stream val- 
leys extending out into the adjacent plains. 

The effects of the climatic change would be regional 
and thus the pattern of erosion and deposition would 
be similar over a wide region. On the other hand, the 
boundaries of the area of deposition might be rather 
sharp and controlled by the major topographic elements 
and by boundaries of wind systems (see p. 66). 

The protracted depositional episode (Oligocene and 
Miocene) cannot, however, be ascribed simply to a 
single brief period of climatic change. If such a change 
were the only control, rapid filling following the change 
would be succeeded by a long period of equilibrium and 
concluded by an even longer period of slow erosion as the 
supply of elastics from the uplands decreased. There- 
fore, if the climatic hypothesis is generally correct, one 
or more modifying factors must also have operated. The 
most probable factors are: 

1. Small supply of material relative to the area of 
deposition. The stream profiles then would be adjusted 
very slowly. The evidence of numerous hiatuses and 
cut-bank erosion within the Chadron suggests, however, 
that the streams were never very greatly out of equi- 
librium and that the supply of material for deposition 
was an incidental factor. 

2. Regional uplifts. Downstream parts of the chan- 
nels would be above grade and would actively downcut. 
Deposition would then cease or slow as the knick points 
shifted upstream. The overall consequences of such 
changes are somewhat difficult to visualize but it seems 
probable that the streams would come to equilibrium 
more rapidly rather than more slowly. 

3. Overloading by volcanic ash in post-Chadron 
times. The increase in ash-falls in late Chadron and post- 
Chadron time was undoubtedly a factor in Oligocene 
and Miocene deposition. It could be the controlling 
factor only if the amount of ash increased progressively, 
since o) the streams were never far from equilibrium, 
and b) the streams would tend to come to equilibrium 
with the amount of ash in their load and any great 
reduction in amount of ash would result in trenching. 
The percentage of ash does increase in the later Chadron 
and through the Oligocene and Miocene, but it seems 
unlikely that this increase is solely responsible for con- 
tinued deposition. 

4. Progressive climatic change. Further decrease in 
rainfall would decrease volume of the streams, both 



66 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



absolutely and relative to available load. With an 
abundant supply of ash largely independent of regional 
climate and with progressive decrease in stream dis- 
charge, continued deposition would be expected. Such a 
progressive overall deterioration of climate is indicated 
by: a) paleobotanical evidence; b) increasing dominance 
of a savanna fauna during the later Tertiary; and c) 
general characteristics of the sediments. 

We conclude, therefore, that climatic change ini- 
tiated deposition in the Oligocene, that it was the pri- 
mary factor in continuance of this deposition, and that 
the supply of volcanic ash was a major but accessory 
factor in later Oligocene and Miocene deposition. The 
fluctuations in deposition then may have been the re- 
sult of either climatic fluctuations imposed on the gener- 
al trend or of fluctuations in supply of ash. 

Climatic Patterns of the Eocene and Oligocene 

The distribution of fossil plants, (Dorf, 1955), of 
invertebrates, and of vertebrates, as well as petrologic 
evidence, indicates that late Eocene climates were warm 
and equable. The temperature differential between the 
Equator and the North Pole must have been low. Under 
such conditions hemispheric wind current systems 
would be weak, of small horizontal extent, and much 
influenced by local or regional temperature differentials. 1 

Assuming the present prevailing westerlies, the 
Eocene jungle-forest environments in the intermontane 
basins are extremely difficult to account for as the 
basins at present are arid or semiarid. All available 
evidence indicates that during the Eocene the moun- 
tains maintained, in general, approximately their pres- 
ent elevations above these basins (although the entire 
area was nearer sea level) so that an explanation based 
on changes in local or regional topography is inadequate. 
If, however, the westerlies were much weaker, local tem- 
perature differences between the Western Interior and 
the two major adjacent seaways, the Gulf of Mexico and 
Hudson's Bay, might be expected to produce a mon- 
soonal climate. 

The area of the southwestern and central United 
States must have been quite warm during the summer 
and thus would function as a thermal low. (Trewartha 
[1954, p. 99] describes the occurrence of such a summer 
thermal low at present). The Gulf of Mexico would be 
somewhat cooler, and Hudson's Bay or the Arctic would 
be considerably cooler. They would become thermal 

1 Communication from Dr. David Fultz, Department of 
Meteorology, University of Chicago: "A number of qualitative 
considerations, both theoretical and empirical, such as the observed 
seasonal differences in circulation between summer and winter, 
suggest that the smaller the general horizontal temperature differ- 
ential in a rotating convective fluid system like the atmosphere, the 
weaker will be the currents and the smaller the horizontal dimen- 
sions of the predominant current systems. The more this is the 
case, the more such systems will be influenced by purely local 
temperature gradients. Laboratory experiments over a wide range 
indicate by comparison with the present 20-30° C between tropical 
and polar regions that if the differential were, say 2°C, the current 
systems would be of the order of size of a few degrees of latitude." 



monsoonal highs. A summer monsoon, with prevailing 
northerlies, would bring moist air in from Hudson's Bay 
to cause heavy monsoon rains throughout the Northern 
and Middle Rockies. (There was probably also a south- 
east monsoon across Texas, but that does not enter into 
the present problem.) Heavy rains probably fell on all 
places above 1000 ft. elevation, as they do in the Punjab 
today. 

The winters were probably dry and cool, with almost 
no wind. It is possible that the slightly stronger winter 
westerlies may have modified the winter monsoon, but 
no evidence of this is known at present. 

Bradley (1948) noted the evidence of alternately wet 
and dry seasons in the middle Eocene Green River 
Shales, and suggested that the climate was monsoonal. 
He lacked, however, the supporting evidence given by 
modern knowledge of conventional dynamics, and sup- 
posed the summers to be dry and warm, the winters cool 
and wet. The present hypothesis makes necessary warm, 
wet summers and cool, dry winters. 

Additional evidence of prevailing northerlies in 
Wyoming is offered by the distribution of volcanic ash. 
Ash constitutes a high proportion of the mass of middle 
Eocene sediment in the Green River and Washakie 
basins, and a much smaller proportion of middle and 
late Eocene sediment in the Uinta Basin to the south. 
Houston (1964, p. 18) considers that the volcanics of the 
Green River and Washakie Basins "may have come 
from the Absoroka source but the petrography is not 
definitive. . . . Petrographically these units equate to 
the acid breccia of the Yellowstone-Absaroka source, 
but in fine-grained rocks this far from source one might 
expect some loss of heavier more mafic minerals espe- 
cially if the major transport was aerial." We consider 
that the Yellowstone-Absaroka district is the most 
probable source, because: (1) it is less than half as 
distant as the next nearest possible sources in Oregon 
and Nevada; (2) the proportion of ash decreases notably 
from the Green River Basin southward to the Uinta 
Basin, as it should if the source lay to the north, and 
should not if the source lay to the west; and (3) as 
Houston has pointed out, the petrography of the vol- 
canic sediments in the Green River and Washakie 
Basins is compatible with Yellowstone-Absaroka vol- 
canics. If this is the case, the tremendous volume of ash 
in the two basins, 300 miles from the source, indicates 
that the prevailing winds during late Eocene time were 
northerly. 

The authors feel that the evidence justifies the hy- 
pothesis that during late Eocene time the climate was 
warm and equable, with a low temperature differential 
between the Equator and the North Pole. Consequent 
weakening of the prevailing westerlies permitted a mon- 
soonal circulation to develop, with prevailing northerlies 
bringing moist air from Hudson's Bay and the Arctic 
toward the thermal low in southwestern United States. 

The rainfall pattern produced by a monsoonal cir- 
culation with prevailing northerlies would be strikingly 
different from the present rainfall map. By analogy with 
the present situation in the Punjab, one may presume 
that little precipitation would occur below elevations of 
1000 ft. This means that most of the Central Lowlands 
would be dry plains or even desert, analogous to Delhi 
or the Sind, while the northern High Plains and the 
Wyoming basins enjoyed heavy summer rainfall. There 
is, of course, no evidence bearing on Eocene climates in 
the Central Lowlands, but the deeply-weathered, lat- 
eritic pre-Chadron surface in South Dakota certainly 
suggests warm, highly seasonal rainfall. 

The general lowering of temperatures in the Oligo- 
cene and early Miocene (Dorf, 1959, and preceding sec- 
tions of this paper) would represent an increase in the 
Equatorial-Polar temperature differentials and thus 
would increase the strength and extent of the hemi- 
spheric wind system. The monsoonal system would be 
greatly modified or destroyed, and replaced by pre- 
vailing westerlies and a cyclonic storm system similar to 
the recent pattern. Rainfall distribution would then 
come to approximate the Recent with a marked decrease 
in total rainfall on the eastern slopes of the central and 
northern Rockies. This area is, of course, precisely that 
in which stream regimen was profoundly changed at the 
beginning of Chadron time.