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Striations: the Stone Groove Truth!!
Erosion of the land surface by the last great ice sheet that engulfed the region is well represented in Maine. Large-scale features include U-shaped valleys, cirques, stoss-and-lee or ramp-and-pluck topography, and glacially striated and polished bedrock surfaces.
Striations are scratches or grooves on a rock surface made by debris at the base of the ice as it is dragged over the rock by the flowing glacier. The striations and grooves are generally smaller scale features in the family of erosional marks left by the glacier. However, these scratches on the bedrock surface can provide a better understanding of where the ice flow was directed and how it changed areally and over time.
Early geologists, including C. T. Jackson, first State Geologist of Maine, referred to the grooves and scratches as "diluvial marks," in reference to the deluge described in the story of Noah from the Book of Genesis in the Bible. After Louis Agassiz provided field evidence for continental glaciation in 1840, other geologists began finding the evidence, too, and by the latter half of the 19th century, most geologists recognized the scratches as erosional features produced by glaciers and not flowing water.
A Whole Lotta Scratchin' Going On
Striations and glacial grooves can be found in many places in Maine. As part of the Maine Geological Survey STATEMAP program, two excellent examples of sites with multiple striation sets derived from different ice-flow directions were recorded this summer during field mapping in the Farmington 7.5-minute topographic quadrangle. The first site is in the town of Farmington, located on Granite Heights Road, which can be reached by travelling about one mile northeast on Maple Avenue from its junction with Route 2. Granite Heights Road is on the left. There are several striated bedrock pavements about 0.1 mile after making the turn onto Granite Heights, on either side of the road where a cul-de-sac (Heritage Circle) joins the main road.
The multiple sets of striations at this locality are not found at any one outcrop, but rather are preserved on several different outcrops. The oldest and most deeply gouged set trends from 155 - 175 degrees and is the dominant set seen on surfaces, with the largest grooves and directional indicators present. A younger set that crosscuts the stronger set ranges in trend from 190 - 209 degrees. Finally, a third wider-ranging set is present in a few places. It is faint and cuts both earlier sets with azimuth trends of 45 (225) - 80 (260) degrees; it may be that this set is actually two separate sets, 45 (225) - 65 (245) and 70 (250) - 80 (260), as they are not found together on any one surface. At two sites here, striations with azimuth trends of 125 (305) degrees at one location and 115 (295) at another are found. No ice-flow directional indicators were found associated with this trend.
The second site is located in the town of Temple on Intervale Road, which parallels Temple Stream. It can be found by travelling west on Route 43 out of West Farmington to Temple Village. Turn left onto Intervale Road, and after about 0.75 mile Mitchell Brook Road will be on your left. Continue over bridge crossing Henry Mitchell Brook and 0.1 mile on left will be second house. The single story building is built on a bedrock pavement that is strongly striated, showing three separate striation sets all with ice-flow direction indicators.
The striations on this outcrop include a strong set, the most robust and deeply eroded, with a trend range of 135 - 145 degrees (the 135 trend appears younger by cross-cutting the 145 trend, and may be a later shift from the 145 trend). A younger trend, 154 - 162 degrees, cuts the strong earlier trend. Another younger faint set, trending 115 - 127 degrees, is superimposed on the older 135 - 145 and the other younger 154 - 160 sets.
Yeah, Like the Grooviest History, Baby!
The striations at these two sites record changes in ice-flow direction. Their age relationships are determined by strength of each set (robust, deeply gouged being strongest and hence most likely the oldest; faint, light scratching being younger), and by the cross-cutting or superposition of one set by another. At the first site in the Cascade Brook valley east of Farmington, the main phase of the Laurentide Ice Sheet is represented by the 155 - 175 degree striation trend. This trend is strong, and is common throughout much of southwestern Maine. The next younger set (190 - 209 degree trend) may reflect the initiation of deglaciation and the influence of local topography on the waning ice sheet as the ice was gutted out of the Androscoggin River and Kennebec River valleys by the glacial sea that flooded inland during deglaciation. The youngest striation set, the 45 (225) - 80 (260) degree trend, may be a response to the ice margin being drawn down locally when the sea was in the Farmington area in the Cascade Brook and Sandy River valleys.
The second site in the Temple Brook valley west of Farmington can be interpreted with a similar sequence of ice-flow events as that found at Cascade Brook valley. The strong 135 - 145 degree trend may represent the onset of Laurentide ice advancing through the region, its flow trend controlled by local topography of the Temple Stream valley, which is 135 - 140 degrees. The next younger set, 154-162 degrees, is parallel to the trend of striations found at higher elevations and elsewhere in the quadrangle, and may represent the regional flow at the glacial maximum, similar to the 155 - 175 strong trend at the Cascade Brook site. However, here in the lowest part of Temple Stream valley that regional trend isn't the strongest striation set due to the local topographic influence. The youngest set, 115 - 127 degrees, like the youngest set in Cascade Brook valley, may be a response to draw down of the ice when the margin was in Farmington in contact with the ocean during the late stage of deglaciation of the region. The ice would have thinned considerably by late deglacial time, and tongues of ice would have occupied the valleys where the ice flow would be controlled by local topography and the marine influence at the ice margin.
For general information on glacial erosion processes and landforms (1), and papers on striations and ice flow in northern Maine (2), and Quebec (3), see below:
1) Bennett, Matthew R., and Glasser, Neil F., 1996, Glacial geology: Ice sheets and landforms: John Wiley and Sons, Chichester, England, 364 pages.
2) Lowell, T. V., Kite, J. S., Calkin, P. E., and Halter, E. F., 1990, Analysis of small-scale erosional data and a sequence of late Pleistocene flow reversal, northern New England: Geological Society of America, Bulletin, v. 102, p. 74-85.
3) Occhietti, S., Parent, M., Shilts, W. W., Dionne, J-C., Govare, E., and Harmand, D., 2001, Late Wisconsinan glacial dynamics, deglaciation, and marine invasion in southern Quebec, in Weddle, T. K., and Retelle, M. J. (editors), Deglacial history and relative sea-level changes, northern New England and adjacent Canada: Geological Society of America, Special Paper 351, p. 243-270.
Text and photos by Tom Weddle, with apologies to Austin Powers, baby!
Originally published on the web as the August 2002 Site of the Month.
Last updated on October 6, 2005
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