Question: when were most of the effigy mounds built
[{'answer': '350 - 1300 CE', 'provenance': [{'wikipedia_id': '4439322', 'title': 'Effigy mound', 'start_paragraph_id': 1, 'start_character': 234, 'end_paragraph_id': 1, 'end_character': 246, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '4439322', 'title': 'Effigy mound', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 1190, 'bleu_score': 0.9441661848199481, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17424780586881841116}, 'section': 'Section::::Abstract.'}]}]