The question mark is used in these cases:

1. at the end of any sentence that is a direct question:
   
   
   Where is the contact between the two formations?
   
   
   
2. after every direct question of a series that makes up a single sentence:
   
   
   When trying to identify any hand sample, we must ask ourselves these questions: what is the grain size? the texture? the colour? the mineralogy? the rock type?
   
   
   
3. enclosed in parentheses, to express a doubt about the correctness of a statement:
   
   
   The quartzite of the Ludding Formation is overlain by fine-grained sandstone of the (?) Packs Formation.
   
   Note:
   
   (?) is a query expressing doubt.
   ? is a punctuation mark, placed at the end of a question.
   
   Position the (?) carefully in order to define exactly what is questionable:
   
   
   (?) Lower Devonian
   questions the entire statement.
   
   (?)Lower Devonian
   questions only Lower.
   
   (?) Silurian–Devonian
   questions both ages.
   
   (?)Silurian–Devonian
   questions only the Silurian age.
   
   Silurian–(?)Devonian
   questions only the Devonian age
   
   (?)[Upper Bathonian]–Callorian
   questions only Upper Bathonian
   
   
   
   
4. to indicate missing digits:
   
   
   Stanley Greenshields (1909–198?).

The question mark is not used in the following:

1. after indirect questions:
   
   
   The geologist asked the students which formation the samples came from.
   
   
   
2. if the sentence is technically a question but actually a request or command:
   
   
   Will you please return my manuscript immediately.