thick gre w on the surface of a clay-rich soil, becoming the first life on land
some 2.6 billion years ago.
The stromatolite colonies formed from layers of cells topped by photo-
synthetic organisms that multiplied using sunlight and supplied the lower lay-
ers with nutrients. Stromatolites are only indirect evidence of early life,
however.They are not the remains of the microorganisms themselves but only
the sedimentary structures they built. Early stromatolite fossils exist in 3.5-bil-
lion-year-old sedimentary r ocks of the Towers Formation of the Warrawoona
Group in North Pole, Western Australia. The region was once a tidal inlet,
overshadowed by tall volcanoes that erupted ash and lava, which flowed into
a shallow sea. Thunderclouds hovered over the peaks, and lightning darted
back and forth. Furious winds whipped up high waves that pounded the
basaltic cliffs of the coastline.
Farther inland, hummocks (rounded piles) of black basalt flows, still
steamy from their latest eruptions, dominated the landscape. The rotten egg
stench of sulfur was pervasive. Frequent downpours fed tidal streams that
meandered onto a flat expanse of glistening gray mud before reaching the sea.
Elsewhere, scattered shallow pools containing highly saline water periodically
evaporated, leaving behind a variety of salts. Often, a flood tide washed across
the mudflat,shifting the sediments and replenishing the brine pools.Figure 16Stromatolites
west of Logan Pass,
Glacier National Park,
Montana.
(Photo by R. Rezak,
courtesy USGS)
Historical Geology