teria might have arisen in the earliest stages of biologic evolution, probably
about 3.5 billion years ago. Iron-metabolizing bacteria might have been respon-
sible for the great banded iron formations laid down some 2 billion years ago.
Modern stromatolites reside in the intertidal zones above the low-tide
mark. Their length reflects the height of the tides, controlled mostly by the
gravitational pull of the Moon.The oldest stromatolite colonies of the North
Pole region grew to great heights, with some attaining lengths of more thanFigure 17Late Miocene
marine diatoms from the
Kilgore area, Cherry
County, Nebraska.
(Photo by G.W. Andrews,
courtesy USGS)
Historical Geology