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Home > Explore! > Bedrock Geology > Baxter State Park > Glacial Geology > Erosion

The Geology of Baxter State Park and Mt. Katahdin

Features produced by glacial erosion

Striations

glacial striations
Figure 9
Gravel, boulders, and other debris lodged in the base of a moving glacier scrape and abrade bare bedrock surfaces, producing outcrops which have a smooth polished surface and grooves and scratches called striations. If the compass directions of the striations are recorded on a map as a series of lines, a picture of the direction of glacier motion maybe gained. Figure 9 shows an outcrop with well developed striations.

In Baxter State Park many outcrops of sedimentary rocks have striations, whereas Katahdin granite and Traveler rhyolite have weathered surfaces on which striations are commonly not preserved. Bedrock surfaces of igneous rocks which have been buried by glacial sediments and only recently exposed have well preserved striations, as in the vicinity of Ripogenus Dam. Several outcrops of sedimentary rocks along the Patten Road show striations, especially on the north side of the road at Hurricane Deck. A few outcrops near the Pattern Road just north of Horse Mountain are striated as are several outcrops of sedimentary rocks along the road from Trout Brook Farm northward to Second Lake Matagamon.

Striations measured in northeastern North America indicate a general northwest to southeast glacier motion originating somewhere near Hudson Bay in Canada. The direction of glacier motion in Baxter State Park evidently diverged somewhat from this general direction, probably as a result of the obstruction of the mountains in the area.

U-shaped valleys

U-shaped valley
Figure 10
Glaciated valleys have a characteristic shape which distinguishes them from stream eroded valleys. Looking up or down glaciated valleys, they are seen to have a broad U-shape, with steep valley walls and broad valley bottoms. Figure 10 shows such a valley which leads from the Northwest Basin. Nearly all the large valleys in Baxter State Park have the same shape, as, for example, the valley of Nesowadnehunk Stream east of Doubletop Mountain, Witherle Ravine, and the long valley leading from the Roaring Brook campground to Russell Pond and on to the South Branch Pond campground.

Cirques

The most spectacular result of glaciation in Baxter State Park are the many well developed and well preserved cirques, which were the source areas of valley glaciers. All of the well known basins, North Basin, Great Basin, Northwest Basin, and so forth, are cirques; each of which nourished a valley glacier. A cirque differs from the head of a normal valley in that it does not become narrower upstream, but rather broadens into a semi-bowl shape. This difference may be seen in Figure 11A and Figure 11B, which show the typical cirque-shape of North Basin and the narrow headwaters of Howe Brook on Traveler Mountain.

North Basin cirque
Figure 11a
Head of valley of Howe Brook
Figure 11b
aerial view of Mt. Katahdin
Figure 12

Many of the cirques in Baxter State Park are shown in Figure 12, a high level aerial photograph, and the rounded shapes of the heads of the cirques are apparent. Also clearly visible in Figure 12 is the long U-shaped valley, shown in Figure 10, which leads from the Northwest Basin cirque.

Cirques are formed principally by a form of weathering which is called frost-wedging. Water from melted snow trickles down to the underlying bedrock. When it freezes in joints and other openings in the rock, the expansion dislodges a fragment of rock which becomes incorporated into the moving ice of the glacier and is carried down the valley, eventually ending as part of the moraine at the end of the glacier. A continuation of this process widens the valley, producing the characteristic cirque shape. The result is a very steep headwall which retreats as long as an active glacier exists in the cirque. The cirque is deepened by the ice as it moves down the valley, ripping off fragments of bedrock from the whole valley floor, and carrying them to the end of the glacier.

An interesting feature, which may be a very small cirque, occurs on the west side of South Turner Mountain, nearly at the summit. Much smaller than other cirques in Baxter State Park, the South Turner cirque gives a picture of the larger cirques as they may have appeared many thousands of years ago. Perhaps the feature is not a cirque at all, but may be the result of some kind of landslide.

As mentioned above, present day glaciers occur at middle and low latitudes where elevations are sufficient to provide the necessary climatic conditions. For the same reason, cirques in Baxter State Park, source areas of former glaciers, occur only at high elevations, and most of them are found in Katahdin granite. The principal exceptions to this general statement are a few poorly preserved cirques in the Traveler Mountain area.

Ar�tes

Probably the single most spectacular feature of the scenery of Baxter State Park is the Knife-Edge, another type of erosional feature which is the result of valley glaciation. Such narrow, irregular ridges are called ar�tes, and are formed by the widening of valleys by valley glaciers. The retreat of the headwall and valley walls of a cirque because of rapid weathering and erosion results in the formation of an ar�te if the high lands above the cirque are narrow. Assuming that North Basin now contained a valley glacier, an ar�te would not form until the headwall had retreated westward across the Tableland and the Tableland was reduced to a ridge as narrow as the Knife-Edge.

Hamlin Ridge also is an ar�te, formed between the two valley glaciers which existed in North Basin and the Great Basin cirques. Few other ar�tes occur in Baxter State Park because the cirques are too widely spaced and the higher elevations in Baxter State Park consist mainly of broad uplands, such as the Tableland (see Figure 12).

Roches mountonn�es

An interesting product of glacial erosion is a group of smoothed, sometimes striated, ridges of bedrock which are similar in size and shape to a whale's back. Such features have been called roches moutonn�es, because they bear some resemblance to a herd of sheep, especially if seen from a distance. The Northwest Basin Trail follows a large roche moutonn�e between Lake Cowles and Davis Pond. Sever smaller "sheeprocks" occur in Northwest Basin.


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Last updated on January 11, 2008