What are some examples of metaphors in "David" by Earle Birney? Explanations of them would be very helpful.

Here are a few metaphors from the poem "David"
Lines 35-37

We crawled astride
The peak to feast on the marching ranges flagged
By the fading shreds of the shattered stormcloud. 

"Feast" is a common metaphor--'feast your eyes on this.' It compares taking in an impressive sight to eating lots of delicious food.
 
More interesting is the reference to "marching ranges flagged" by clouds. This line compares the long mountain range to an army, marching in line and floating battle standards in the air. It gives a visual image of the nature of the mountain range, and it also gives some insight into the Bob and David's view of the various mountains as opposition to be defeated.
 
This concept is revisited in lines 89-90:


Hundred feet we fought the rock and shouldered and kneed
Our way for an hour and made it.


Lines 49-52

...There it was too that David Taught me to read the scroll of coral in limestoneAnd the beetle-seal in the shale of ghostly trilobites,Letters delivered to man from the Cambrian waves

The coral in the limestone, the beetle-seal, and the ghostly trilobites are all fossils preserved in the sedimentary rock that David and Bob pass. The references to reading "the scroll" of fossils, "letters" from the Cambrian period, describes the wealth of information about ancient pre-history that can be gleaned from the fossils once you decode their 'language.'
Lines 138-139

The glistening hair of a creek on the rug stretchedBy the firs, 

This is another visual metaphor. Picturing a creek as a hair on a green rug gives a sense David and Bob's height in the mountains; they are so far up that entire geographical features look as small and thin as hairs.