JOHN CLARK 



ASSOCIATE CURATOR, SEDIMENTARY PETROLOGY 



WEATHERMEN 



to the past 



"H 



ey, doc, I found a crocodile!" 

 "Fine! Where'd you find him?" 



"Right up there." Kenny's sweat- 

 streaked face, grinning excitedly, poked 

 over the top of a barren rock ledge. "In 

 that pond near the top of the hill." He 

 pointed to a thin, greenish layer in the 

 brown clays of a badlands ridge crossing 

 the hot Utah desert around us. The 

 gritty surface clay crunched to dust be- 

 neath our boots as we scrambled hastily 

 up to the site. 



Kenny had indeed discovered a croc- 

 odile — bones of a baby croc scarcely 

 three feet long. And it had certainly 

 died in a pond. However, the croc- 

 odile's life had ended and the shallow 

 jungle pond had filled forty million 

 years ago. Now we saw it as a scat- 

 tering of small, dark brown objects in 

 a greenish sandstone. We knew that 

 only in the airless waters of a jungle 

 pond would rotting vegetation produce 

 enough carbon to rob the sand's iron 

 minerals of their oxygen and turn them 

 green. Had there been only land, or a 

 cleaner pond, the residual colors would 

 be rust-brown or ocherous. To my en- 

 thusiastic young assistant, the dry, dusty 

 present had disappeared, and he saw a 

 small, dead crocodile floating among 

 the ferns and leaves of a little pond, 

 shaded by the lush jungle growth that 

 he knew must once have been. 



This was as it should be, because for- 

 mally we were engaged in a study of 

 ancient climates. It was our fascinating 

 job to peer into the past, to the time 

 of forty million years ago, and deter- 

 mine what the weather had been. Was it 

 warm or cool? Were the winters frosty, 

 or did the temperature never drop to 



Page 6 February 



freezing? What was the annual rainfall? 

 Did the rains come as a few cloud- 

 bursts, or in an annual rainy season, 

 or were they well distributed through- 

 out the year? Which way did the pre- 

 vailing winds blow? Were they con- 

 stant, like tropical trade winds; or sea- 

 sonal, like the monsoons; or variable, 

 like the winds around Chicago? 



The first step in determining ancient 

 climates is to reconstruct the geography 

 of the period. We already knew that here 

 in northern Utah an enormous lake had 

 existed for millions of years. A large 

 river had brought water from the moun- 

 tains of Colorado one hundred miles 

 east and discharged it across flat plains 

 to the muddy eastern shore of that lake. 



The ancient mud flats are now shales, 

 and the sand in the channel of the an- 

 cient stream is now sandstone that 

 makes winding, sandy ledges across the 

 present dusty flats. Simple measure- 

 ment of the sandstone ledges showed us 

 that the old channel had been 100 yards 

 wide and six to eight feet deep. This 

 is more than four times as big as the 

 modern White River, which drains the 

 same mountain area today. The coars- 

 est sand in the sandstone was pea-sized 

 gravel; experiments show that water 

 must be moving two to three miles per 

 hour in order to roll pebbles this size 

 along its bed . Four times as much volume 

 of water as the modern White River, 

 moving at least twice as fast, would 

 mean about ten times as much rain as 

 now falls in western Colorado, unless 

 that rain came all during one short sea- 

 son each year, so that more of it would 

 have run off. And that led us to the 

 next question : was the rain seasonal, 

 or well distributed? 



The answer to this was revealed by 

 the finely layered shales that had once 

 been the muddy shores of the ancient 

 lake. In these thin layers of shale 

 we found beautiful fossils of tropical 

 leaves and tiny fishes. But between the 

 shaly layers were layers of gypsum and 

 other salts, and the tops of many shale 

 layers were indented by oddly-shaped 

 angular cavities. These were molds of 

 salt crystals that had formed while the 

 shale was still a mud flat and had dis- 

 solved away again during the ages since 

 they were buried. Gypsum and salts 

 can only crystallize out of lake waters 

 that are actively evaporating. This 

 meant that the old lake suffered fre- 

 quent droughts, when the surface evap- 

 orated and salts crystallized out on the 

 recently-exposed mud flats. The rain- 

 fall was thus seasonal, and the runoff 

 great; we figured that the rainfall in the 

 mountains was probably about five 

 times what it is at present, with alter- 

 nate seasons of wet and very dry. This 

 would be like the monsoons on the south 

 flank of the Himalayas, in Pakistan. 



Which way did the prevailing winds 

 blow? Ash from volcanoes gave us the 

 best clue to that. Volcanic "ash" is 

 actually small, angular grains of vol- 

 canic glass; with a microscope, one can 

 recognize it no matter how much sand 

 and mud are mixed in. Naturally, the 

 bulk of the ash from a volcano settles 

 out down-wind, like the tail of a weather- 

 vane. The closer one approaches the 

 volcano, the more ash and the larger 

 particles one finds. We found that the 

 amount of ash increased northward, 

 from very little in Utah to considerable 

 amounts in sediments the same age in 

 southwestern Wyoming. The ash must 

 have come from the great Yellowstone 

 Park volcanic centers, and the prevail- 

 ing winds blew from the north, not from 

 the west as they do today. 



Gradually, bit by bit, a picture of the 

 ancient climate emerged from the rocks 

 and fossils of its time. It had been a 

 monsoonal climate. The northerly 

 winds blew during summertime, and 

 brought in rains that dropped prob- 

 ably 50 or 60 inches of moisture on the 

 mountains. Winters were dry, still, and 

 only moderately cool. Probably there 

 was never any frost. 



