Tuesday, May 22, 2012

Day 4 - The Ordovician & Silurian Periods

Monday, my Geology of the Hudson Valley class examined some Ordovician and Silurian rocks exposed in Ulster County.  No pictures, unfortunately, since it was raining most of the day (pouring in the afternoon).

After the supercontinent of Rodinia rifted apart, this area became a continental shelf with carbonate deposition during the Cambrian and into the Ordovician.  This started to change with the approach of the Taconic Island Arc (see Day 3).

We started down by the Hudson River and examing Ordovician sandstones and shales representing turbidites - relatively deep water shales interbedded with graywacke sandstone submarine landslide deposits.  We looked at some examples of sole marks and graded bedding in the sandstone layers.  These deposits formed as the island arc approached (presumably triggered by earthquakes along the subduction zone associated with the island arc).  The collision of this island arc folded and thrust faulted these shales and sandstones toward what's now the west (we also looked at a couple of faults).  In Ulster County, however, we were far enough from the action that these rocks weren't metamorphosed.

From the River, we drove west to the base of the Shawangunks at the Trapps - a popular rock climbing area known as the Gunks.  There we saw the angular unconformity between the Ordovician shales and the Silurian conglomerate - the Taconic unconformity representing the period of uplift and erosion of the Taconic Mountains.

Over the Shawanungunk Ridge, we stopped at an overlook to the west and spoke about the Rondout Valley where Early Devonian carbonate rocks overlie the older rocks and then grade upward into the sediments of the Catskills.

In the Rondout Valley we examined a few more Silurian rock units - the High Falls Shale, Binnewater Sandstone, and Rondout Formation.  The Rondout Formation is the most important of the three since two of its members were mined for natural cement in a narrow belt from Rosendale to Kingston (there's a book there).  After lectures about the Rosendale cement industry and the D&H Canal (they are closely tied together), we visited a couple of mines in the Rosendale and Kingston area.

Again, no images since it was a torrential downpour by the end of the day and we were soaked to the skin by the time we wrapped things up.  There's also a lot more I could write about the geology (and history) of these rocks but it will have to wait - teaching a 9-hour field course every day is tiring!

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