Geology Reference
In-Depth Information
In his hallmark contribution to geology, Steno adopted guiding principles for interpreting
the history of rocks that he intended to be so simple, clear, and transparent that no one
could dispute them. The first simply states that the layers at the bottom of a pile of sediment
were laid down first, and therefore were oldest. The second holds that sedimentary lay-
ers are deposited horizontally. These simple principles allowed him to start piecing togeth-
er the story of the Tuscan landscape. Defining how to determine the relative age of strata
and past events opened the door to deciphering earth history. It was Steno's distinction
between primary and secondary rocks—between crystalline rocks he thought were made at
the initial Creation and layered rocks that formed later from detritus eroded off the original
rocks—that set the stage for the development of a geological time scale.
By the spring of 1668, Steno's trips into the hills convinced him that the ancients were
right about the nature of fossils. That summer he submitted his findings about fossils and
Tuscan geology to the church's censors, who routinely vetted the theological acceptability
of scholarly discoveries, opinions, and interpretations about the natural world. Although
this meant his
Dissertation on Solids Naturally Enclosed in Solids
wasn't published until
the following year, Steno did not need to worry about meeting Galileo's fate. His interpret-
ation of geological evidence as faithfully recording the biblical flood placated the church
even though he broke with prior tradition to interpret earth history through studying rocks
and fossils rather than scripture.
In contrast to the fanciful theories of his better-known contemporaries, Steno's interpret-
ation of how Noah's Flood shaped the Tuscan landscape was rooted in field observations.
After laying out how fossils got into rocks, he described the sequence of events he read in
the hills around Florence.
He concluded there were six periods in earth history that corresponded to the biblical ac-
count. He found no fossils in the lowest, and therefore oldest, layers. So these rocks formed
right after the Creation, when water covered the world. Before the creation of life, sedi-
mentary rocks lacking fossils settled out in this primeval sea, laid down as horizontal strata.
As these newly deposited rocks emerged to form dry land, Steno thought subterranean fire
or water ate out huge caverns in the underlying rock. When these great caves collapsed to
produce valleys, it triggered Noah's Flood as the seas rushed down to fill the new lowlands.
After more sediment settled in the new, lower-elevation sea the whole process repeated,
