Allagash Falls Geology

Photo by Kyle Marvinney

Introduction

The Allagash Wilderness Waterway is a magnificent stretch of lakes and rivers that spans 92 miles across northern Maine, beginning at Telos Lake and extending downstream to the village of Allagash. This is a popular waterway for paddling enthusiasts who seek solitude in an unspoiled natural setting. Excellent campsites, abundant wildlife and excellent fishing make any trip on the Allagash a memorable experience.

General geology of the Allagash River area

map showing extent of Seboomook formation

Figure 1

Maine's geological landscape is varied and complex, the result of repeated and intense geological processes that operated over many millions of years. In contrast to this complexity, much of northwesternmost Maine is underlain with a remarkably uniform sequence of sedimentary rock units collectively known as the Seboomook Group. Named from exposures near Seboomook Dam in western Maine, rocks of the Seboomook Group extend nearly continuously from the Rumford area to Madawaska, and on into Canada (Figure 1). Units of the Seboomook Group consist mostly of dark gray slate and light brown sandstone, although in this area of Maine slate predominates. Another description of rocks of the Seboomook Group is found in Bald Mountain, Washington Plantation, Maine. The alternating layers of sandstone and slate define many of the ridgelines along the lower Allagash that follow a northeast-southwest alignment.

Allagash Falls

Usually after many days on the river, canoe travelers are confronted with the awesome obstacle of Allagash Falls, announced by a persistent low rumble that seems somehow out of place while paddling among the meandering islands that make up the reach just above the falls. The once complacent river becomes a roaring torrent of white that plunges 30 feet into a series of pools (Figure 2) (Figure 3). What, the traveler must wonder after many miles of river with only a hint of ledge outcroppings, is holding up the falls?

Figure 2

Figure 3

At this locality on the Allagash, the rocks of the Seboomook Group consist of very thickly bedded sandstone. These thick, resistant beds succumb to the erosive forces of the river slowly and have held the falls here for perhaps thousands of years following the melting of the last ice sheet.

The following photographs illustrate some characteristics of the rocks at the falls.