[1]A “strike” measurement is expressed as so many degrees east or west
of north or south. For a diagram illustrating the dip and strike of
a rock layer see Figure 3.
[2]This is one of the three major rock groups or families. The first
consists of igneous rocks, including granite, syenite, and basalt,
which were formed by solidification of molten rock-material. The
igneous rocks are ancestors of the other two rock families; they
form over 90 percent of the outer 10 miles of the Earth’s crust. The
second family, consisting of sedimentary or layered rocks including
shale, sandstone and limestone, is composed of pieces and grains and
other materials from all the families of rocks. In addition,
sedimentary rocks are formed also from lime secreted by marine
plants and animals or chemically precipitated from sea water, or by
the accumulations of shells. The third family, metamorphic rocks,
including gneiss, schist, slate and marble, were igneous or
sedimentary rocks that have been subjected to heat and pressure in
the presence of mineral-forming solutions. Metamorphic rocks
generally look different from the rocks from which they formed,
because the original minerals of the rock have been changed and
reoriented.
[3]The actual remains are usually not preserved in their original state
but are represented by molds and casts. Picture an ancient sea. The
sea bottom mud slowly hardens around a shell. Water then seeps
through the hardened mud and dissolves the shell leaving an open
space where the shell once was. This open space is a mold. If the
mold is filled a copy of the original shell is formed. This is
called a cast.
[4]The relative rather than the absolute age of the rocks can be
determined from a study of their fossil content. These fossils are
compared with collections from various places in the world where the
standard geologic time scale assigns them a place (see Fig. 4). The
Park rocks were deposited during the Ordovician Period. How is a
standard geologic time scale put together? Several geologists first
worked out the sequences of rocks according to the Law of
Superposition in Great Britain and neighboring parts of Europe. When
systematic collections of fossils were made from these layers and
arranged according to age it was found that certain fossils
occurring in rocks in distant areas were identical and occupied the
same relative age position. These fossils were considered to be of
the same relative age. Fossils found in the Park can be compared
with these reference fossils and a relative geologic age can be
assigned to them. Absolute ages can be determined in some cases by
the use of rates of decay of radioactive elements and in general
these ages agree with the relative ages derived through the use of
fossils.
[5]The capitalized syllable is the accented syllable.
[6]An index fossil is used to date the rocks in which it occurs. A good
index fossil must be abundant, widespread and easily recognized. Its
vertical range is restricted to a small number of rock layers,
therefore the geological span of life of a good index fossil is
usually short.
[7]Chitin is a colorless horny substance similar to the material which
makes up fingernails.
[8]The black color is due to an abundance of finely divided organic
(plant and animal) material within the rock.
[9]A quartzite is either a metamorphic or sedimentary rock consisting of
fragments of the mineral quartz (SiO₂) which are cemented together
by silica (quartz). The combination of quartz fragments held
together by quartz cement creates a very hard rock which oftentimes
will break across the fragments rather than around them. The
quartzites of the Park area are primarily of a sedimentary origin.
For a description of the three major rock groups, of which the
sedimentary and metamorphic groups are two, see footnote, D.A.R.
State Park, page 6. A dolostone is a sedimentary rock composed of
fragmental, concretionary, or precipitated dolomite (a mineral of
chemical composition, CaMg(CO₃)₂) of organic or inorganic origin.
[10]The black color is due to the inclusion of finely disseminated
carbonaceous material (animal and plant remains) within the rock.
[11]This splitting or cleavage was produced after the layers had
hardened into rock. The cleavage planes were produced when the rocks
were subjected to pressures too great to withstand. In some places
these cleavage planes do not parallel the layers.
[12]According to the basic geologic law, the Law of Superposition,
younger rocks (those deposited last) are always found resting on
older rocks (those deposited before the younger). The only time that
this is not true is when either breaks (faults) or folds in the
earth’s crust place the layers in an inverted order, as in the case
here cited.
[13]The fault plane of a high-angle fault forms a large angle (generally
from 30 to 90 degrees) at its intersection with an imaginary
horizontal plane. The plane of a thrust fault, or low-angle fault,
forms a small angle (generally less than 30 degrees) at its
intersection with an imaginary horizontal plane.
[14]This is the Burlington till (Stewart, 1961) and was deposited from
the Burlington Ice Lobe during its period of wasting. The till is a
hodge-podge mixture of clay, sand and pebbles and is usually brown
in color.
[15]A kame is a mound or ridge of poorly sorted (sometimes well-sorted,
that is, made up of all the same sized particles) water deposited
materials. Most kames are ice-contact features; that is to say, the
materials which make up the kame were deposited in contact with a
glacial ice surface. The Mt. Philo kame may be the filling of an
ice-free area during the final melting of the glacial ice.
[16]Delta is the name of the fourth letter of the Greek alphabet, the
capital form of which is an equilateral triangle. The
triangular-shaped tract of land formed by the deposit of river
sediment at river mouths is named for the triangular shape of the
capital Greek letter delta.