Geography Reference
In-Depth Information
58
Figure 2.12. A somber day at Finger Mountain. Here is where Ferrar determined that the dark dolerite
he had been finding on the moraines was an intrusion rather than a lava flow. As can be seen, the
dolerite magma intruded along a bedding plane of the light-colored Beacon sandstone, producing
a sill, the thick central band of the outcrop. Fracturing during cooling of the sill produced columnar
jointing, the vertical grain visible in the face of the sill. A large offshoot of the sill diked up along the
right skyline and then flattened into a higher sill (the bottom of which can be seen as the flat bot-
tom of the triangular face at the shoulder to the left of the summit). A graceful arcuate dike shot off
the bottom of the sill, raising slivers of sandstone only slightly before the magma solidified.
matrix of mud. When one is looking for fossils, often the best rocks for preservation are
the ones made of mud, the shales, which come to rest in low-energy environments. But
these sandstones, from what Ferrar could see, had no shaley interbeds.
Then Ferrar let out a whoop. About one hundred feet up the spur, he found a thin,
black seam in the sandstone, maybe one-quarter of an inch thick, running maybe twenty
feet out the face. He broke out pieces of the sandstone along the seam and split them with
his hammer, making quick work of all the rock that could be easily pried loose, but no
luck. The black material looked coal-like, the sort of deposit that would come from or-
ganic material, plants probably, but no impressions of leaves or other possible fossils could
be seen. He guessed that heat from the overlying dolerite sill had cooked these rocks to
the extent that any vestiges of fossils that might have been there had been destroyed.
