Question: name the agent of erosion that produce hanging valley
[{'answer': 'glacier', 'provenance': [{'wikipedia_id': '32416', 'title': 'Valley', 'start_paragraph_id': 34, 'start_character': 19, 'end_paragraph_id': 34, 'end_character': 26, 'bleu_score': 1.0, 'section': 'Section::::Glacial valleys.:U-shaped or trough valley.\n'}]}, {'provenance': [{'wikipedia_id': '32416', 'title': 'Valley', 'start_paragraph_id': 52, 'start_character': 0, 'end_paragraph_id': 52, 'end_character': 570, 'bleu_score': 0.8901415508087751, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1568708655737036439}, 'section': 'Section::::Hanging valleys.\n'}]}]