Question: when was the canadian shield formed what era
[{'answer': 'Proterozoic Eon', 'provenance': [{'wikipedia_id': '6230', 'title': 'Canadian Shield', 'start_paragraph_id': 4, 'start_character': 116, 'end_paragraph_id': 4, 'end_character': 131, 'bleu_score': 1.0, 'section': 'Section::::Geographical extent.\n'}]}, {'provenance': [{'wikipedia_id': '6230', 'title': 'Canadian Shield', 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 1127, 'bleu_score': 0.913555182725125, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17739029047032722576}, 'section': 'Section::::Geographical extent.\n'}]}]