During this landslide, rocks fall because and then build up at the base of the cliff. a) of a friction force b) of the force of gravity c) of the constant movement of the earth d) they are heavier than the soil supporting the

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.