This image from Horse Cave, Kentucky, shows a formation made from many stalagmites combined into a single mass.
The description states:
MAMMOTH ONYC COLUMN
MAMMOTH ONYX CAVE, HORSE CAVE, KY.
This enormous mass of cave decoration rises as an onyx barrier through two elevations of the cavern passages, and is estimated to have required over ten millions of years to develop to its present proportions.
–Some of the color shown is natural, some is enhanced through the use of colored footlights. This card dates from the 1950s.