Elgin's Unconformity
I set this earthcache up a while ago, when I had a hockey tournament in this end of town, but never actually put together a listing.
What is an angular unconformity? Normally, sedimentary rocks are deposited in horizontal strata as in the Philipsville cliff. There, sands washed and sorted on Paleozoic sea coasts more than half-a-billion years ago were cemented into layers.
There are three families of rocks. The sedimentary family includes sandstone and limestone formed and usually found at the Earth's surface. The igneous family includes the granites of Westport Mountain and Rock Dunder, formed deep beneath a mountain range and then raised to the surface. But the rock bands you see in the lower half of the Elgin unconformity are of neither the sedimentary family of rocks nor igneous. They are metasedimentary gneisses and paragneisses of that third and incredibly diverse family called metamorphic rocks. These are rocks that have been changed by conditions of heat, pressure and time; conditions found deep beneath mountain ranges like the Himalyas or Grenvilles. There is no doubt that the families and ages of the rocks in the upper and lower parts of this Elgin road cut do not "conform". However there is a question about the exact kind of rock and its origin in the lower part of Elgin's unconformity.
Elgin Unconformity
There is an almost 500 million year gap between the overlying Cambrian sandstones and the underlying Precambrian gneisses. This gap is the erosional period during which the once mighty Grenville mountains were eroded down to a mere remnant of their former glory.
The most interesting feature of Elgin's unconformity is the in-between, probably the least obvious zone. If you climb to the top of the sloping metasediments, you will find a thin layer of sandy, gravelly rock called a basal conglomerate, a special stratum of pebbles and sands. Basal conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments. On that pre-Cambrian peneplain beneath Elgin, everything that was dissolvable was washed and wafted away by stream and wind over eons of time. Hard sands and pebbles of quartz persisted to tell of a time when there was little free oxygen in our Earth's atmosphere.
Then about 600 million years ago, Paleozoic seas flooded this sterile and flattened landscape. Winds and waves of the Cambrian Period reworked this ancient sandscape into horizontal sandstone, layers we call the Nepean Formation (or Nepean Fm for short). As the Paleozoic sea persisted during the Ordovician Period, it left massive layers of Beekmantown dolomite on top of the Nepean sandstone, and continued eastward.
Definitions:
- Strata: In geology and related fields, a stratum (plural: strata) is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers.
- Unconformity: An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe any break in the sedimentary geologic record.
Questions to answer:
- Take a look at the layer of conglomerate. Pull out your ruler (Don't forget to bring one!) and measure how thick it is. If you find it varies, find an average.
- How tall is the rock outcrop, starting from the bottom of it, not the bottom of the ditch, of the grass, or whatnot?
- Look at the outcrop. How far up the outcrop does the 500 million year gap occur?