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A Pleistocene Ecosystem
by Wesley Gordon
page 9

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Descriptive Data

Hundreds of thousands of cubic yards of clay, sand, and gravel have been taken from the Irvington deposits by workmen using bulldozers and power shovels. Most of the walls of the excavations are vertical, and many of them are more than 100 feet high. These walls contain clues about how and when particular strata were deposited. How a paleontologists, Donald E. Savage of the University of California employed the clues at Irvington and noted and studied what he found and how he interpreted his findings are worth our attention.

Amateur paleontologists are searching for fossils in the exposed strata. 

First, he collected his data. Along a line running northeast to southwest across Locality V3604, Savage investigated about 270 feet of exposed strata. When he completed his study, he recorded the findings in tabular form. He described the strata that contained fossils and indicated were the fossils were found. He described the different types of sediments found at different elevations. He also indicated distinct cross-bedding. The exposures contained information that will, if correctly interpreted, provide a base on which you may reconstruct scenes depicting biotic and abiotic events at Irvington.

This information is useful when we attempt to answer such questions as: Were all the animals buried at the same time? Were some beds especially rich in fossils? Did the ancient streams flow more rapidly at some times that at others? Did the streams change direction from time to time? Did earth movements raise or lower the area during the time the sediments were being buried? While looking over the paleontologistís data table (Figure 8), notice how he arranged his data.

Such an arrangement helps you to visualize the geologic conditions more clearly. Suppose the data in the columns had been presented like this:

  • Greenish-gray mudstone, 8; gray friable sandstone, 15; cobble gravel, 4-6; tough gray sand, 7-9; sandy pebble-cobble gravel, 8-12; and so forth.

 You probably would have been tempted either to rearrange the data or to stop reading. But there is no reason why the data could not be presented clearly in another way. Draw a vertical section of your own, using a sheet of paper large enough to permit a scale of 1/8 inch to represent 1 foot. At this scale the section should be about 34 inches long. Color the different strata according to the descriptions given by Savage.

Figure 8: Description of sediments and Fossil Remains, Locality V3604, Irvington District, Fremont, Alameda County, California.

Lithology

Thickness
(in feet)

Fossil Remains

(Vertical Section of Quarry)    
Fine to medium grained sandstones and mudstones, deep tan with brown-red area; soft (uncemented), showing some cross-bedding. 70    
Sandy cobble gravel (cobbles vary from 64 to 256 mm in diameter), poorly sorted (mixed with sand) 48   Small fragments of bone
Soft, gray sandstone, distinctly cross-bedded 23   Mammuthus, mammoth, Equus, horse; Camelops, Irvington Camel
Pebbly and gritty sand (pebbles vary from 4 to 64 mm in diameter), with channeling.  The cross-section of a complete stream channel-fill is exposed 20 +/- Mammuthus; Equus; Camelops; Tetromeryx, antelope; Canis, dire wolf
Soft, gray sandstone 10 +/-  
Loose, medium-grained, gray-green sand (1/4 - 1/2 mm) 1 +/- rodents
Soft, gray sandstone with carbonized (changed to coal, charcoal, or carbon) and limonitized (altered by iron oxides or iron hydroxides) wood fragments 7 +/-  
Loose, coarse, gray-green sand (1/2 to 1 mm) 2 +/- Camelops; Equus; Terameryx; rodents; turtle
Brown-yellow to gray claylike sand with scattered pebbles and pebbles with lens-shaped cross sections; grades downward into tougher (compacted) sandstone 20 +/- All Irvington mammals except some rodents; includes fish, amphibians, turtles, birds, snails, clams, twig imprints, and leaf imprints
Pebble-cobble gravel with lens-shaped cross sections 10   Mammuthus; Camelops; Arctodus, a gigantic bear
Green-gray mudstone 8    
Gray friable sandstone 15    
Cobble gravel 5    
Tough, gray sand 8    
Sandy pebble-cobble gravel 10   Mammuthus; Equus; Camelops
Tough, gray sand (bottom of exposed section) 11   Mammuthus
Total Thickness 270 +/-  

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