What is the fine-grained topic of the following text: The railroad closed the agency there 14 years later when the line between Fort Benton and Big Sandy was abandoned because of a "slip out" along the bluff above the Missouri River's left bank, aggravated by an irrigation system installed on the bench above town in 1976, Jacobsen found. Only return the topic.
Closure and abandonment of a railroad line due to geographical and environmental factors.