A Noah's Ark of fossils in downtown L.A. In La Brea they dig the pits

ON a nondescript parcel of land in midtown Hancock Park here, George Jefferson dons hard-hat and boots beneath a piercing afternoon sun. The same solar rays that are glinting off Porsches and Mercedeses headed for the posh salons of Beverly Hills nearby, are metamorphosing the hardened tar floor of ``Pit 91'' into a gooey mess. Mr. Jefferson grabs the blackened ladder protruding like a straw from this 160-square-foot hole in the ground. And as he descends 13 feet into the earth, he delights that in a few seconds, he can escape the mundane trappings of modern life, for the (literal) trappings of the Ice Age, 40,000 years ago.

``We've got the lower jaw of a saber cat, and the femur [shin bone] of a dire wolf,'' says Antonia Tejada Flores, kneeling with hammer and chisel on the pit's floor.

Bones of all shapes and sizes protrude from a one-square-yard cake of asphalt, cut about six inches deep, and set off by string. Scattered around the confines of Pit 91 are spatulas, paintbrushes, chisels, trowels, hammers, and dental equipment. Under Jefferson's curatorial tutelage, Ms. Flores, a paleontologist (studier of fossils), and two assistants, are scraping and dusting the bones, measuring their placement in a numbered grid. Then the bones are extracted and placed in manila envelopes or plastic bags, with precise measurements scribbled under nine categories.

The placement of each of the bones in layers of tar is crucial to understanding the exact history of the area, so well preserved here at the La Brea Tar Pits, the world's largest deposit of Ice Age fossils.

``Here is a slice of life preserved like no place else in the world; everyone interested in geology or paleontology should visit it,'' says Troy Pewe, a paleontologist at Arizona State University. ``It's internationally known for the quantity and quality of specimens. Every museum in the world gets specimens from there.''

Besides the high number of fossils here - some 10,000 of one species of extinct wolf, for instance - La Brea is distinguished for the diversity of species present and for the high quality of preservation, from rodents to mammoths, because of the properties of tar. Well over a million fossils of over 560 species of plants and animals have been discovered here.

For the fourth consecutive year, the George C. Page Museum is continuing its public excavation of Pit 91, a key Los Angeles summer attraction and the only open fossil dig of its kind in an American city. Work on the pit had progressed steadily from 1969 to 1980 under the direction of the Natural History Museum of Los Angeles County, with funds from the National Science Foundation, the National Endowment for the Humanities, and the Natural History Foundation. Excavation stopped when funding ceased in 1981, but public donations allowed the project to resume in 1984.

THE paleontologists who retrieve, classify, and study the remains of plants and animals that were trapped here by asphalt-rich sediments are recording a complete record of life in this basin between 4,000 and 40,000 years ago. The results, they say when pressed for applications of their esoteric pursuits, help complete a story that has bearings on today's understanding of the environment and climate: What will be the effect of clearing much of the Amazonian rain forests? What levels of CO2 can be accommodated before that gaseous waste product from automobiles affects plants and animals? How can we better manage our natural resources?

Some climatic event occurred here a few millennia ago and made a number of species of plants and animals extinct. What was it?

Of course, that's the scientific side. ``I just like finding the stuff,'' says Ms. Flores. ``Half the appeal of paleontology is the allure of the treasure hunt, to be the first to set eyes on something that hasn't been seen in 30,000 years,'' she says, pointing out the fused, lower-back vertebrae of a dire wolf. With her accumulated knowledge and experience, Flores can identify an entire animal from a one- or two-inch fragment.

Jefferson, assistant curator at the Page Museum, where the fossils are on display, echoes a similar sentiment. ``There is a curious streak in humans that is kind of satisfied by things in the past like these mammoths, extinct sloths, and tapirs,'' he says.

``Ever since I was a kid, I picked up fossils and wanted to know why the plants they had then are not around anymore. To study it all is like traveling through time.''

Contrary to popular beliefs, the ``tar'' pits are not tar at all, and did not act like huge, deep vats of quicksand. Crude oil, which occurs naturally below ground, surfaced through fissures and cracks. The oil evaporated and the remaining asphalt puddled up in stream beds and low-lying areas. Summer's heat dried the streams and warmed the semi-solid asphalt to a gooey liquid.

Camouflaged by dust and leaves, the sticky surface trapped unwary animals like flypaper. Larger animals such as ground sloths, bison, and horses became trapped, luring other carnivorous animals and birds - vultures and condors. During winter, the asphalt became solid, and rain-swollen streams covered them with silt and sand, until summer liquefied the asphalt and reset the trap, forming layer after layer of asphalt, fossils, and sediment.

EXCAVATION has gone on at this site for about 100 years. The larger bones are cleaned and combined like puzzle pieces into composite skeletons for the museum - saber cats, mammoths, tapirs, and sloths being the most well-known. Sediment collected here is saved as well. Later on it is packed in screen baskets and boiled in solvent to reveal microfossils, seeds, snails, small plant remains, and insect parts.

In recent years Pit 91 has revealed previously unknown bats, moles, snails, crustaceans, and microscopic plants called diatoms. In the early 1970s, scientists discovered the only coast redwood fossil ever found in the Santa Monica mountain region.

In 1914, the 9,000-year-old La Brea Woman, the only human skeleton ever discovered in the tar pits, was unearthed at an adjacent pit located in the same compound.

Now in its 10th year, the Page Museum is ``doing more projects with less money,'' according to Jefferson. A full-time staff of 20 is aided by 120 volunteers with public funding from Los Angeles County, and private fund from the Natural History Museum Foundation.

Plans include computerization to be better able to cross-analyze already existing data, and new graphic display digitizers that will save time measuring the positions of fossils within already excavated blocks of asphalt.