<Tr> <Td_colspan="1"> Northern Song </Td> <Td_colspan="1"> </Td> <Td_colspan="6"> Western Xia </Td> </Tr> <Tr> <Td_colspan="1"> Southern Song </Td> <Td_colspan="1"> Jin dynasty </Td> <Td_colspan="6"> </Td> </Tr> <Tr> <Td_colspan="8"> Yuan dynasty 1271--1368 </Td> </Tr> <Tr> <Td_colspan="8"> Ming dynasty 1368--1644 </Td> </Tr>

What physical feature made up much of the northern border of the tang dynasty