Combine the pair of sentences by changing one into an appositive. Remove all unnecessary or repeated words. Set the appositive off with...

• An appositive is a noun or
  a pronoun that is placed beside another noun or pronoun to identify or describe
  it. 


• Appositives often act as
  clarifiers as that provide necessary identification to nouns or pronouns. 
  


• Appositives usually
  follow the words they identify or explain; however, for emphasis an appositive may come
  at the beginning of a sentence.

e.g. Our
teacher, Mrs. Walworth, has received a special
award.

• In addition, a writer may use an
  appositive phrase.  This phrase consists of an appositive and any
  modifiers that accompany the appositive.

e.g.
The Alaska moose, the llargest deer in the world, inhabits the Kenai
Peninsula.

So, to rewrite the above-mentioned sentence
using an appositive, the writer can change the last sentence into this
construction:

Suddenly, the navigator noted
an obstacle straight ahead, a large
iceberg.

[Note that identifying what to use
as an appositive is often easy because one noun or pronoun will be repeated.  The
repeated noun/pronoun is the word to eliminate and then place the descriptive or
explanatory words after the first noun/pronoun.]