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emłutkw in a v^vk\jk\il contest

lar interwds m geologowi tune the polarity ot this tield has changed so that the modem pat tern (posttiYfe m the North orn Hetnisphere and negative in the Southern Hemi sphere) has been cerersed. Processes such as eooling ot Loa. settling oftnagnetic particles durmgsediment depose non and growth ofiron rich nunerals within sedimentary rocks can presen e a record ot the magnetie tield when the twks were tbnned. The tield direetion is ineasured using a nugnetometer. Fotassiuin-argon dating oflavas whose polarity Has been measured m this way gi\es a titnescale ot the chanie ofpolarity tor about the past 12 tnillion vears, This has been e\tended to the becimbnę of the Meso*oie eta hv $tudv ol'magneue anomaltes on the ocean tloor.

I he patiem ot ancien* changes to the earth's polarity m samples through a stratigraphie section is eotnpared with the known pattern ot* the palaeomagnetic polarity lunescale Most rock strata aro not continuous, so that thetr approxiinate oge must be established by fossits or by isotopie dating bełore theit magneticsignalure can be titied to the correct time period.

This tnethod gwes importatU Information on severat pnnuie tossil locahties where the ehionology is alreads reasonably well known tbre\Atnple. sit the hominid site ot Olduval Gocge in Tan/ania, A most signitkwm conuibution hsis been to give age estmiAtes ot the Mivvene sedunents ot theSiwalik region ofnorthem Pakistan. where uummalian tossils. including hominoids, have been tound. The lamus

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ofthe Nam and Obok Paihau Formations m this region a te now know n to be between b and i: miUion \ eats old. As i; pAlsieoniAgnetic transitions e\ist in the sequence between hy and tkb niilhon years s\go* it is possible to esiabhsh an accurate chronology tor this portion ofthe sednem e.

TH, Bromi

S«v «tSO Ctimotfc Ch%n$* ui the post' _VP V T4>. Tossi* <fepostt$ and tłH>u <n\*stijut*ołV tP 187k tśNolutlon of au^uak^pttheeines' tp 231) Aflti Etotutionof ftftriy hum>ms'\P 241)