1)There is written in the Macmillan dictionary that the word 'morning' can be both: countable or uncountable.
Could you give me an example sentence where 'morning' is uncountable?
2) Is 'morning' uncountable in the following sentence?
He sat by the river all morning.
If not, could you explain why there is omitted "the" after "all"?(not 'all the morning' but 'all morning')