Geology

Geology. . , The Matanuska River valley in which you are standing is a structural trough formed by the collision of the North American and North Pacific tectonic plates during the Jurassic and Cretaceous Periods (200-65 million years before present, MYBP), a time when dinosaurs roamed the Earth. This trough was inundated by a shallow sea during the late Cretaceous times (100-65 MYBP) with marine waters depositing shales, siltstones and sandstones of the Matanuska Formation that can be viewed at many locations along the canyon of the Matanuska River between Palmer and the Matanuska Glacier. The fossil remains of a Hadrosaur (duck-billed dinosaur) called 'Lizzy' were discovered in this formation during the 1990's. After the Hadrosaur died, it is speculated that the carcass was carried by water from the shore out into the shallow marine environment where it was entombed and preserved., As the shallow sea subsided, a sinuous river system occupied the Valley bottom during the Tertiary Period (65-35 MYBP) including large swamps and wetlands that provided the abundant organic deposits to form the coal beds of the Chickaloon Formation as well as preserving abundant fossils. Over time, large alluvial fans extruded from the mountains onto the valley floor capping the Chickaloon Formation with gravels that later solidified forming the Wishbone Conglomerate. This resistant layer caps Wishbone Ridge, the sloping ridge three miles to the northwest of the Alpine Historical Park, and also forms the cliffs of Castle Mountain about ten miles further up the Valley. The Chickaloon and Wishbone deposits occupy the perimeter of the Valley and lower mountain slopes., Surrounding the Matanuska Valley are the Chugach and Talkeetna Mountains. From the Park Crag Peak is the most prevalent landmark of the Chugach Mountains to the south and was formed in sedimentary rocks of the Jurassic Period (200-145 MYBP). Behind this mountain, further south, lies the igneous core of the Chugach Range where molten rock was injected into the previously existing mountains which eroded away exposing the resistant granitic intrusive rocks. King Mountain about ten miles further up the Valley provides an excellent example of this process. The Border Ranges fault, a complex of right lateral strike slip faults that fractured and uplifted the Chugach Range during Cretaceous and Tertiary times. Evidence suggests that this fault system has not been active in the recent epoch. Rising to the North are the Talkeetna Mountains, a series of granitic Jurassic age spires, including Granite Peak, the largest in our view. The active Castle Mountain Fault, a right lateral strike-slip fault generally runs along the base of the Talkeetna Mountains. Earthquakes along this fault system are frequent. The surface on which we stand is the product of one of many glaciations during the past 250,000 years, the last one ending about 9,000 years ago., Glaciers bulldoze materials forming hills, ridges, and plains, covering the underlying bedrock with loosely unconsolidated sediments. Large rivers poured from melting ice during the end of the last glaciation depositing the extensive sand and gravel deposits common throughout the Valley including the ground on which we now stand. Winds, for which local residents are so familiar, deposits dust called 'loess' which provides the valuable growing medium on which much of the Valley's farms rely. Paraphrased by Mark Clark from Roadside Geology of Alaska, 2nd Edition, by Cathy Conner.