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   MATHER GORGE FORMATION

Site Locations: (4) 

Widewater/Lift Lock 15, mileage: 13.25                                 (Nearest Access: Old Anglers Inn, mileage 12.62) The builders of the Canal discovered that here was an abandoned channel of the Potomac that could be used as a widened segment of the Canal.  The decision was made to incorporate the channel by protecting it from incursions and undercutting of the river by constructing walls and waste drains.  This saved cutting into the adjoining cliffs. During the 1996 floods, the embankment at the downstream end of the basin was blown out by the force of the water allowing the old channel to be viewed in its pre-Canal form.  The Widewater provided safe mooring for several boats at a time during their transits up and down the Canal.

Mary’s Wall,   mileage 13.85                                                                                   (Nearest Access: Great Falls Tavern, mileage 14.30)                                                                 The view from this location is possibly the most spectacular of any along the Canal.  In the foreground is a secondary channel (meander) of the Potomac that at times may be nearly dry, at others a significant waterway.  The wall was built to protect the Canal from undercutting by the channel during floods or seasonal high water.  Rocky Island separates this channel from the main channel of the Potomac.  At the base of the wall is a pool in the channel called Catfish Hole.  Whether a home for catfish or not, it is a favorite pool for fishing.  The trailside plaque provides an interesting perspective on the influence of the ice age on the quantity of water in the Potomac basin and its erosional effect on Mather Gorge.             

Great Falls Tavern Visitors Center,   mileage: 14.30                               Compared to downstream views of the river, the Potomac appears most placid from this vantage point.  One cause for this is the dam that has been built here to funnel water into the aqueduct that leads to Dalecarlia Reservoir.  But without it, the river flow would still be placid at normal water levels, disguising what unsuspecting boaters will incur just a few hundred feet downstream.  The Potomac declines in elevation about 600' between Cumberland and Georgetown but over 10 percent of that decline is here at the Great Falls.  Note that you have just passed 6 locks from the Angler's Inn; that many being required to keep the Canal "level" with the Potomac.                                      

Blockhouse Point, mileage 21.10                                                      (Nearest Access: Violettes Lock, mileage 22.12)                                                                        (Also observed at the Great Falls Park Virginia Visitors Center)                                                                          The last exposure of the Mather Gorge rock formation upstream.  A particularly scenic location along the Canal with the very erosion resistant rock reaching  the Virginia shore causing the rapids.  A rock wall built here protected the Canal from undercutting by the Potomac.  This is an example of where a natural rock wall jutted out to the river requiring the Canal builders to shave it back to allow sufficient width for the canal and the towpath; witness the drill marks into which black powder was poured.  Otherwise, the Canal is a ditch dug into the riverbank and flood plain or river terrace.

The MATHER GORGE rock formation is a highly complex assemblage of metamorphosed sedimentary rock that has been intruded by igneous rocks and has been in part nearly melted by heat generated at depth.  The original sediments were deposited in a deep oceanic trench of the Iapetus Ocean (pre-Atlantic) through the mechanism of turbidity currents that surged down the continental slope to form a submarine fan.  The member units of this formation are now called metagraywacke (originally muddy sands), mica schist (formerly siltstone), and migmatite (the nearly melted original rock).  The Mather Gorge Formation was thrust out of the Iapetus trench, pushed up and over rocks that had formed on the continental slope and shelf (for example, the Sykesville Formation that is exposed downstream near the Beltway).  It became accreted to the continent at approximately the location where it is now observed.  The sediments comprising the Mather Gorge Formation were deposited during Proterozoic time, between 700 and 600 million years ago.  Intrusions of granodiorite and lamprophyre dikes occurred in the interval between 460 to 360 million years ago.                                                 

Hiking the “Billy Goat Trail” permits close inspection of the rocks:  the earliest deformations by folding and faulting, the textures caused by heat and pressure during metamorphism, the intrusives, ox-bow channel, and the relatively recent erosional sculpting, for example, the potholes.  The Widewater is interesting in that it is a remnant of a river channel, a meander carved from the rock when the Potomac was flowing at this elevation .   The channel bottom was exposed in 1996 when the floods burst the canal wall effectively de-watering the Widewater.  A detailed guide book to the Great Falls Park Virginia has been prepared with photos that presents the Mather Gorge Formation in somewhat more detail (contact is stan@geohelp.com).  A compelling walk for the geologist and a beautiful walk for everyone is the trail along Difficult Run that terminates, as does the stream, at the Potomac River downstream from Mather Gorge. 

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