He rang the RSPCA and inspectors rescued the dog, which was extremely emaciated, with a potentially life-threatening womb infection and a large tumour.
Now the RSPCA is appealing for information from anyone who saw the dog walking or being carried to the gully, which is around a mile up a steep track popular with dog-walkers.
It took an hour for two inspectors and three members of the public to get the dog out of the hole. She was so weak she had to be carried to the van.
Inspector Alison Wilkin-son said: "Someone must have seen something. It's possible she had the strength to walk there earlier on in the day, but whether she walked or was carried, she is in such a state that it's likely she would have been noticed."
Cynthia, who is being cared for at the Halifax animal centre in Wade Street, is light grey, aged five or six, with a docked tail. She was not wearing a collar or ID tag and is not microchipped.
"She's been weaning puppies until very recently. Obviously we're very keen to make sure they're okay too", added Inspec-tor Wilkinson.
The RSPCA will pay for the vet bills and hope to rehome Cynthia once she is well again.
Julie Cockroft, Halifax animal centre manager, said: "She needs an urgent operation, which will be very expensive, and months of aftercare, so any donations would be very much appreciated."
Cheque donations should be made out to RSPCA Halifax and District Branch and sent to the centre at Wade Street, Halifax, HX1 1SN.