‘Yes - the roof ones are... «Famous Five: 16. Five Go To Billycock Hill»

‘Yes - the roof ones are stalactites and the ground ones are stalagmites,’ said Dick.
‘I simply never can remember which is which,’ said Anne.
‘It’s easy!’ said Julian. ‘The stalactite icicles have to hold tight to the roof - and the stalagmite might some day join with the ones above them!’