Within the general science of geology are several branches, to name only a few; paleontology, sedimentology, mineralogy, petrology, stratigraphy, petroleum geology, and structural geology. Each of these branches or specialty-areas contributes basic data for the overall interpretation of the past geologic history of any given geographic area. The historical geologist takes all these clues and attempts to fit the pieces of information together into a picture of past events.
The concept of Geologic Time must be understood before the history of Darling State Park can be unraveled. Usually we think of time in terms of minutes, hours, days, weeks, months and years. The geologist thinks and talks in terms of millions or even billions of years. Time units as short as hundreds of years are impossible to distinguish in the past history of the Earth. When it is realized that the earth is probably 4 to 6 billion (4,000 to 6,000 million) years old, and the record of these years is incomplete, it is easy to understand why the geologist speaks in terms of millions of years instead of years. With modern methods of radioactive dating the geologist hopes for finer time definitions in the future.
In short then, the geologist interprets and puts order into millions of years of history which can only be “read” as recorded in the rocks beneath our very feet. Of course, just looking at the rocks does not magically open the book of geologic history. This pamphlet is designed to sharpen your powers of observation and to help you in your interpretation of these observations.
Figure 2. Thin-section of granite from Burke Mountain. The main minerals seen in this photograph are feldspar (light gray center right, marked with “F”); quartz (whitish, marked with “Q”); biotite (light gray, speckled appearance, marked with “B”). Note the interlocking nature of the minerals which make up this rock. Magnified 15 times, under crossed nicols.
THE ROCKS AND THEIR HISTORY
The most conspicuous rock found in the park is granite.[1] Along the road which winds to the summit of Burke Mountain you will see several outcrops of the white or pinkish biotite granite ([Fig. 4]). This granite is well displayed in the summit parking area and along the trail to the observation tower (Figs. [3] and [5]). A walk down the Bear Den Ski Trail also shows an abundance of granite outcrops (Figs. [6], [7], and [8]).
Figure 3. Speckled granite with inclusion of metamorphic rock. Black specks in granite are flakes of black biotite mica. Metamorphic inclusion, located just above the hammer head shows some reaction with the invading granite. Pieces of metamorphic rock were undermined by and dropped into the granite as it worked its way upward into these rocks. Picture taken a few yards west of the tower on top of Burke Mt.
While looking at some of the above-mentioned photographs, a second family of rocks is discovered (Figs. [3], [5], [6], [7], and [8]; also, Figs. [9], [10], [11], and [16]). In many places these rocks have a layered or banded appearance and in other places large lath-like crystals are common in some of the layers. In some areas these rocks are very heterogeneous in appearance and display distorted layers and profuse development of lath-like crystals (Figs. [12] and [13]). These rocks belong to the second major family of rocks, the Metamorphic rocks. The metamorphic rocks[2] seen in the park were originally sedimentary[3] rocks. These rocks belong to the Gile Mountain Formation[4] which was deposited during the Devonian Period some 300 million years ago (see Geologic Time Scale[5], [Fig. 14]). So much for the two major families of rocks present in the park, the igneous and metamorphic rocks, and how to distinguish one from the other. Let us assume that you can now distinguish between the granite and the metamorphic rocks.