Matanuska-Susitna Valley Geology

The Matanuska-Susitna Valley has been perfectly carved over thousands of years to frame glaciers, mountains, and rivers, and to serve as an entrance to exploration. This valley is located in the southeastern region of Alaska, dipping into the Interior. You’ll pass giant peaks etched away and dropping into cliffs that meet the river basin. Accessing the geological wonders this valley has to offer is as easy as a drive down a paved highway. This area is only an hour from Anchorage and a bridge to remote grandeur. (USGS)
ziplining the Matanuska Valley
ice geology
fly over the Matanuska Glacier

Rock

The Matanuska-Susitna Borough encompasses over 23,000 square miles. (Matanuska-Susitna Borough, Alaska, 2020) In the Mat-Su Valley, geology is shaped by three ranges: the Talkeetna Mountains, the Chugach Mountains, and the Alaska Range. Much of this valley lies atop rock that formed during the age of the dinosaurs. (Jones, 1963, pp. 1-53) Fossils are now embedded in the sedimentary rock, and people fossil-hunt along Moose Creek, looking between rock layers and along the riverbanks for prehistoric gems. (Fossils & Dinosaurs, n.d.)

Additionally, metamorphic rock overlies these sedimentary features, giving the landscape its jagged appearance. (Capps & Mertie, 1927) The crumbling metamorphic rock makes travel across the Chugach and Talkeetna ranges more dangerous for adventurous travelers in the southern region. (Nokleberg et al., n.d.) In contrast, some rock faces expose igneous slabs. These walls see few climbers, leaving quality routes for sport and traditional climbing. (Capps & Mertie, 1927)
The valley is surrounded by the Talkeetna range on one side and the continually growing Chugach range on the other. As soon as you enter the lowland, you look up to glacier-carved walls spotted with giant batholiths, formed from cooled magma deep in the Earth’s crust. (Capps & Mertie, 1927) The glaciers responsible for these carvings are the Matanuska and Knik Glaciers. These rivers of ice were once thousands of feet higher and longer than they are today. (Glaciers in Matanuska-Susitna County Alaska, n.d.)

Solidified Water

Glaciers ground through the valley with the weighted force of ice and gravity. They moved along, gaining and losing mass as the seasons changed. They scraped the basal ground layer beneath them and the walls of what we now see as a canyon landscape. Deposits of rock and old moraine line the toe and edges of the glacier. This moraine lies over sub-levels of ice and gives way to feelings of walking on the moon.

These slow-moving glaciers scraped away at the rocky mountain sides, hence leaving behind the current crumbled terrain. This terrain is composed of clay, rock, and fine-grained sediment known as rock flour. (Rock flour, 2024) This rock flour can form into thixotropic mud, a non-Newtonian substance. (Perret et al., 2021) As a result, the substance has the characteristics of cornstarch and water. This can create a fun, hands-on science experiment, a spa-like experience, or, if not given proper awareness, a life-threatening hazard.
Explore Alaska Glacier

Liquid Water

Water takes the easiest pathway through the ice and over the mud, deluging into the Matanuska River. The waters are thick with a gritty rock flour, constantly churned by the raging waters, creating a grey appearance. The river flows into the valley, braiding itself with the easily eroded shorelines.
As glaciers melt, they influence the river; similarly, the river shapes the shorelines. This fast-moving water also undercuts giant walls of conglomerate rock. (Curran & McTeague, 2011) Boating down the river may be the best way to see these spectacular rock vistas.
Alaska braided river
Magma

Multiple points of confluence exist in the river from the glacial melt. (Geomorphology and bank erosion of the Matanuska River, southcentral Alaska, 2011) This exacerbates the difference in rock flour density across water sources. (Quantifying periglacial erosion: Insights on a glacial sediment budget, Matanuska Glacier, Alaska, 2009) Further, it supports a variety of watercolors in rivers and creeks. (Lawson, 2017) As you round the corner from Caribou Creek, you can catch a view of the Lion’s Head. This valley figurine is a volcanic plug. (Trainer, n.d.) It can be spotted by its resemblance to a prowling lion.

Volcanism once opened the earth here, oozing magma that slowly cooled layer by layer until a plug formed. (Capps & Mertie, 1927) There is a 2,000-foot plummet off the southeast-facing wall of the head. (Lawson, 1982, pp. 279-300) Once upon a time, the Matanuska Glacier had sheared this face off of the plug as it moved down the valley thousands of years ago. (O’Farrell et al., 2009) You can hike up this volcanic terrain and reach sprawling views of the Matanuska Glacier spilling out between Mt. Wickersham and Mt. Marcus Baker.
Exploration

There are many perspectives to choose from when exploring the valley. You can take the bird’s-eye views from a helicopter seat and get the bigger picture, or cruise along the highway in your vehicle. Maybe you’ll step over the rocky moraine to unearth quartz and olivine scattered about. There are many opportunities to admire the many facets of geology in the Mat-Su Valley.

References

(2020). Matanuska-Susitna Borough, Alaska. United States Census Bureau. https://www.census.gov/quickfacts/fact/table/matanuskasusitnaboroughalaska/INT100223

Jones, D. L. (1963). USGS Mesozoic 8595, Matanuska-Susitna Borough, Alaska, USA. USGS Professional Paper 432, pp. 1-53. https://www.mindat.org/paleo_loc.php?id=7691

(n.d.). Fossils & Dinosaurs. Alaska Division of Geological & Geophysical Surveys. https://dggs.alaska.gov/popular-geology/fossils-dinosaurs.html

Capps, S. R. & Mertie, J. B. (1927). Geology of the Upper Matanuska Valley, Alaska, with a Section on the Igneous Rocks. U.S. Geological Survey Bulletin 791. https://doi.org/10.3133/b791

Nokleberg, W. J., Plafker, G. & Wilson, F. H. (n.d.). Geology of south-central Alaska. https://pubs.usgs.gov/pp/1574/report.pdf

(n.d.). Glaciers in Matanuska-Susitna County Alaska. MountainZone.com. https://www.mountainzone.com/mountains/alaska/matanuska-susitna-ak/glacier/

2024). Rock flour. Britannica. https://www.britannica.com/science/rock-flour

Perret, D., Locat, J. & Martignoni, P. (2021). Thixotropy of soft clay: A review. Engineering Geology 287. https://doi.org/10.1016/j.enggeo.2021.106097

Curran, J. H. & McTeague, M. L. (2011). Geomorphology and Bank Erosion of the Matanuska River, Southcentral Alaska. U.S. Geological Survey Scientific Investigations Report 2011-5214. https://doi.org/10.3133/sir20115214

(2011). Geomorphology and bank erosion of the Matanuska River, southcentral Alaska. U.S. Geological Survey. https://doi.org/10.3133/sir20115002

(2009). Quantifying periglacial erosion: Insights on a glacial sediment budget, Matanuska Glacier, Alaska. U.S. Geological Survey. https://doi.org/10.3133/70030704

Lawson, D. (2017). Characteristics and Origins of the Debris and Ice, Matanuska Glacier, Alaska. Journal of Glaciology. https://doi.org/10.1017/jog.2016.118

Trainer, F. W. (n.d.). Surficial geology of an area adjoining the edge of the Matanuska Valley, Alaska. https://www.usgs.gov/publications/surficial-geology-area-adjoining-edge-matanuska-valley-alaska

Capps, S. R. & Mertie, J. B. (1927). Geology of the upper Matanuska Valley, Alaska, with a section on the igneous rocks. U.S. Geological Survey Bulletin 791. https://doi.org/10.3133/b791

Lawson, D. E. (1982). Mobilization, Movement and Deposition of Active Subaerial Sediment Flows, Matanuska Glacier, Alaska. The Journal of Geology 90(3), pp. 279-300. https://doi.org/10.1086/628680

O’Farrell, C. R., Heimsath, A., Lawson, D. E., Jorgensen, L., Evenson, E., Larson, G. & Denner, J. (2009). Quantifying periglacial erosion: Insights on a glacial sediment budget, Matanuska Glacier, Alaska. Earth Surface Processes and Landforms. https://doi.org/10.1002/esp.1885