Monday, April 5, 2010

4-06-10 Percy Quadrangle; the surficial & bedrock geology of the Percy Peaks 4-4-10 (in progress)

(Map 1.) This is the geologic map of the Percy quadrangle in northwestern New Hampshire which accompanies Randolph W. Chapman's Petrology and Structure of the Percy Quadrangle, New Hampshire that was his doctoral thesis at Harvard and published in 1946. I'm including it in the blog because I was just up on the Percy Peaks (see below). They're rather striking mountains and on this map are located in the kidney-shaped, pink area in the upper center of the above map. I'm also including this piece in the blog because it's well written and explains clearly the complex White Mountain geology. The map is intriguing. It looks like an abstract painting. It's difficult to read because the print is so small so I've included other maps below. I'll refer to as the "Chapman Map" from now on. And I'm only going to quote bits of Chapman's narrative, enough so you can grasp the complexity of the geology itself and understand how challenging it was for Chapman (with lots of help from colleagues) to figure out the truly ancient, complicated history of New Hampshire geology and explain it to the rest of us.

(Map 2.) This is the 1996 series Percy Peaks U.S. topographical map with the same scale and area as Chapman's colored 1946 geologic map. I drew the black line here to represent a portion of the pink, kidney shaped area surrounding Percy Peaks on the colored map.

(Map 3.) This section of the 1996 topographical map details the terrain just north of the Percys and it is marked at the top left corner with a black line to show the bright orange area in the upper left corner of Chapman's Map. In the center, upper right half of the map there is a dark shape made up of closely crowded contour lines that represent the small 'nub' with the dark green, coniferous cap (consisting of balsam fir and spruce trees) that's in the next photo and located just north of the Percys .

The photo was taken from the summit of North Percy looking due North. You can identify this small peak with the dark green, coniferous "cap" on the topographical map by its shape and "feel" . This small peak is the western terminus of Whitcomb Mountain that is a long ridge running east-west. Keep in mind that the balsam-spruce zone starts at roughly 2100 feet and the highest summit is 2827. We can use this peak as a reference to understand how Chapman gathered and organized his data. We can also use it to build a better sense of the relationship between bedrock and surficial geology and local soils, plants, and fauna in a remote forest area like the Percy quadrangle.

This is the view north from North Percy using a normal lens and the small peak with the dark green cap we were looking at is now at the correct position and scale in the middle distance on the right. The long, high ridge on the left side of the photo conforms to the half circle drawn with a black line on the topographical map (above) and represents a large 'ring dike complex' which will be explained in Chapman's narrative.

These are the Percy Peaks with the North Peak in the background as seen from the southwest side, near Groveton, NH. You can see how intriguing they are and why Chapman was drawn to this area and be the first to study the geology. The steep ledges are on North Percy's west and soutwest face. They're safe to climb when dry. The north summit is a riot of blueberries in late July-early August.

(Map 4.) This is a geologic map copied from Marland Billings' 1949 Paper "Mechanics of Igneous Intrusion in NH" which is long but brilliant. This map and legend will help us see what igneous intrusions look like in several areas of the state. Billings writes in this paper, "In fact, it seems probable that new Hampshire contains a greater variety of large intrusive forms than any area of equal size in the world."

The legend for the Billing's map shows numerous granites around the state. Billing's doctoral thesis was the North Conway quadrangle and half way down the list you see the symbols for the Moat Volcanics. Billing's research on this particular volcanic intrusion is one of his brilliant contribution to our understanding of the geology of the White Mountains.

The Percy Quadrangle is at the very top in the A4 slot and if you click on the map to make it larger you can see the features designated on the Chapman Map with colors. For instance, the area around the Percy Peaks is now kidney shaped but with coding indicating it as Biotite granite, or Conway granite as it is commonly referred as. The small, half circle at the top of the m map is Syenite.

Probably the most interesting feature of the Percy Quadrangle is the circular pattern of mixed minerals that looks like an near-perfect segment from a Florida orange on the Chapman Map and on Billing's map we see that it, too, is Syenite. This is a large ring dike complex that underlies the Kilkenny Wilderness area and Mt. Cabot in particular. If you look at the whole New Hampshire map you see the Syenite in numerous "Ring Dike" configurations. The most dramatic is the almost perfect circle underlying the Ossipee Mountains split between the Winnepesaukee and Mt. Chocorua quadrangles.

I'm going to quote from Chapman's narrative starting with his Abstract:

"The Percy Quadrangle in northern New Hampshire embraces an area of rugged topography maturely developed by stream action and subsequently modified by continental glaciation. The region is drained chiefly by the Upper Ammonoosuc River which flows generally westward into the Connecticut River.

"Strongly folded and metamorphosed micaceous quartzites and schists of the Albee formation and somewhat younger amphibolites of the Ammonoosuc volcanics constitute the oldest (upper Ordovician?) rocks of the region. These have been intruded by silicic plutonics representing four distinct magma series: (1) Lost nation group of the Highlandcroft magma series (upper Ordovician?), (2) Oliverian magma series (middle Devonian?), (3) New Hampshire magma series (upper Devonian?), and (4) White Mountain magma series (Mississippian?).

"Ring dikes and stocks, composed of syenite and granite of the White Mountain magma series, form features of particular interest in the quadrangle and are believed to have been emplaced by right-fracture stoping or cauldron subsidence. In some cases the process appears to have involved wholesale intrusions of magma along clean-cut, ring-shaped or arcuate fractures; in other instance arcuate shatter zones apparrently developed and these were intruded by multitudinous small dikes which gradually replaced the country rock."

Chapman, at the time he wrote this dissertation, was a graduate student at Harvard studying with Billings who visited him in the field and assisted in the data collection for this study. Billings has been mentioned in this blog a number of times and, along with myriad generations of Goldthwaits, his name is legendary in the arenas of glacial geology, bedrock geology and the surficial geology of New Hampshire and the White Mountains. Katherine Fowler-Billings, his wife, has equal status in these arenas. She, too, helped Chapman on the the Percy and Rumney (NH) quadrangles, and completed mapping her own quadrangles including the Monadnock Quadrangle located in the southwest corner of NH. In 1996 she published a moving reminiscence of her career in geology called Stepping-Stones: The Reminiscences of a Woman Geologist in the Twentieth Century. One gets the impression from her autobiography, all of the existing literature, that this community of geologist/scientists was a collaborative, closely knit group.

To be continued......

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