A large boulder towers over hikers on the beach at Captain Cook State Recreation Area north of Nikiski at the end of the Kenai Spur Highway. The boulders, common to the area and called glacial erratics for their random placement across the landscape, were deposited by glaciers from the west side of Cook Inlet, according to geologist Dick Reger.
Clarion file photo
Over the ages, the Kenai Peninsula has had one makeover after another as glaciers have waxed and waned over Southcentral Alaska like phases of the moon.
During each of the many ice ages that have come and gone over the peninsula, glaciers have dislocated dirt and rocks over long distances as they grind and carve their way through the landscape. As the weather has warmed between each of these ice ages, glaciers have receded to reveal some unusual landscapes to be explored.
One such landscape can be found at Captain Cook State Park north of Nikiski, where the last ice age left an exotic collection of boulders known as glacial erratics scattered below the park’s bluffs and across its beaches.
A glacial erratic is a rock that has been carried and deposited by glacial ice away from its place of origin. In Captain Cook State Park, visitors can find glacial erratics ranging from gravel size to as big as a small house.
Glaciers carried the rocks and boulders from the Tyonek area on the west side of Cook Inlet and deposited them on the west side of the Kenai Peninsula between 17,000 and 18,000 years ago, said Dick Reger, a consultant for Alaska Division of Geological and Geophysical Surveys.
“They almost all are rock types not typical of the rock types they’re in,” Reger said, referring to the erratics found at Captain Cook State Park. “They’re not at all related to the local bedrock, they’re quite different.”
On the west side of the Kenai Peninsula, most of the local surface rock is sedimentary, rock formed when sediments accumulate and harden. Most of the erratics found at Captain Cook State Park are igneous, rocks formed when molten rock cools, or metamorphic, rocks formed when sedimentary rocks are exposed to extreme pressure and heat deep below the earth’s crust.
“There’s quite a story going with those things, and it’s all there to be seen with the naked eye,” said Ed Berg an ecologist at the Kenai National Wildlife Refuge who teaches a geology class at Kenai Peninsula College.
The park’s varied collection of rocks also includes sedimentary and conglomerate rocks, rocks formed when a collection of individual stones have become cemented together to form a larger rock.
“They’re all kinds of rock,” Reger said, referring to the park’s erratics. “We even find coal out there.”
Reger said one of the most common rock types found among the park’s erratics are granitic, igneous rocks that cool from molten rock while below the earth’s surface.
So how did rocks formed beneath the earth’s surface on the other side of the inlet end up scattered on the beaches of Captain Cook State Park?
As the land uplifted to form mountains on the other side of the inlet, granitic rocks were also uplifted. As the land around and on top of the granitic rocks eroded, these igneous rocks were exposed.
“And the glaciers came along and picked them up and carried them over,” Reger said. “Most of the big ones landed on top of the glacier from rock avalanches in the mountains over there.”
Glacial erratics can be found in many places over the peninsula, but what makes Captain Cook State Park unusual is the number of erratics that can be found there.
“There are so many of them, there are thousands of them,” Reger said. “And a lot of them are really good size.”
There are so many erratics at Cook Inlet State Park, because as ice receded away from the peninsula during the last major ice age, the foot of a glacier stopped and paused at the park for a long period, and in the meantime left a large deposit.
The ice found in a glacier is continually moving forward, but as it’s moving forward it’s also melting and evaporating. If the rate at which the glacier is moving forward is the same at which it it is melting and evaporating, the end of the glacier will stagnate in one location while continuing to deposit rocks and debris at its terminal end like a conveyor belt.
To get the best view of Captain Cook State Park’s erratics, be sure to visit the park during low tide.
Patrice Kohl can be reached at firstname.lastname@example.org.
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