Glacial erratic boulders of the Puget Sound region

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Many glacial erratic boulders (often simply called glacial erratics) can be found in the Puget Sound region as far south as the Yelm area where the Puget Lobe of the glacier reached its maximum extent.

The Pleistocene ice age glaciation of Puget Sound created many of the geographical features of the region, including Puget Sound itself,[1] and the erratics are one of the remnants of that age.[2] According to Nick Zentner of Central Washington University Department of Geological Sciences, "Canadian rocks [are] strewn all over the Puget lowland, stretching from the Olympic Peninsula clear over to the Cascade Range."[3] Erratics can be found at altitudes up to about 1,300–1,600 feet (400–490 m) in the Enumclaw area,[4] along with kames, drumlins,[5] and perhaps also the unique Mima mounds.[6] The soil of Seattle, the state's largest city, is approximately 80% glacial drift, most of which is Vashon glacial deposits (till),[7] and nearly all of the city's major named hills are characterized as drumlins (Beacon Hill, First Hill, Capitol Hill, Queen Anne Hill) or drift uplands (Magnolia, West Seattle).[3][8] Boulders greater than 3 meters in diameter are "rare" in the Vashon till,[2] but can be found, as seen in the table below.

List of erratics[edit]

Map all coordinates using: OpenStreetMap 
Download coordinates as: KML · GPX

A few of the larger or otherwise most notable erratics can be found in this table. More erratics are noted in the area-specific lists in the navigation box.

Name and description Height County Image
Airport Boulder, at Martha Lake Airport Park in Martha Lake, said to be "one of the largest glacial erratic boulders in urban King, Snohomish, and Pierce counties",[9] is approximately 20 feet (6.1 m) long on its longest axis and about twice a man's height. The erratic is composed of greenstone,[10] and has long been used for bouldering (rock climbing), with at least four ascent routes.[11][12]

47°51′53.7″N 122°14′10.7″W / 47.864917°N 122.236306°W / 47.864917; -122.236306 (Airport Boulder)

12 feet (3.7 m) Snohomish Martha Lake Erratic 2015 1.jpg
Arroyo Park erratic, in Arroyo Park in Bellingham, is approximately 10 feet (3.0 m) tall and 17 by 23 feet (5.2 m × 7.0 m) in extent. It is mostly diorite with granodiorite dikes.[13][14]

48°41′54″N 122°28′45″W / 48.6983°N 122.4793°W / 48.6983; -122.4793 (Arroyo Park erratic)

10 feet (3.0 m) Whatcom
Big Rock is a 8-foot (2.4 m) tall glacial erratic in the city of Duvall. A Duvall road, a park, and several businesses are named after it.[15][16] The rock, and two non-native sequoias adjacent to it probably planted by area pioneers, are a local landmark.[17] The erratic lies in what is said to be the smallest King County park, 20 by 70 feet (6.1 m × 21.3 m) in extent, that barely contains the rock and sequoias.[18] The two lanes of Big Rock Road used to split into a wye around the rock, until a shopping center was built nearby in the 1990s.[17][18]

47°43.542′N 121°59.189′W / 47.725700°N 121.986483°W / 47.725700; -121.986483 (Big Rock (Duvall))

8 feet (2.4 m) King Big Rock erratic in Duvall north face.JPG
Big Rock, once known as the Rock of Ages, a greenstone erratic and a landmark on Main Street in Coupeville, at one time considered for a city conservation easement[19][20][21]

48°12′38″N 122°41′14″W / 48.21056°N 122.68722°W / 48.21056; -122.68722 (Big Rock (Coupeville))

30 feet (9.1 m) Island
Dabob Bay erratic, a non metamorphosed conglomerate erratic on the shore of Dabob Bay, possibly part of Chuckanut Formation from Whatcom County 80 miles (130 km) away.[22]

47°48′N 122°48′W / 47.8°N 122.8°W / 47.8; -122.8 (Dabob Bay erratic)

Discovery Park beach erratics

Four or more erratics on beach below Discovery Park. Largest is 15.33 feet (4.67 m) high, 69.5 feet (21.2 m) in circumference.[23]

47°40.124′N 122°25.210′W / 47.668733°N 122.420167°W / 47.668733; -122.420167 (Discovery Park beach erratics)

15.33 feet (4.67 m) King Eastern Discovery Park beach erratic, yardstick.jpg
Fort Townsend State Park erratics are two erratics in Fort Townsend State Park. One is granite, probably from the British Columbia Coast Range; the other basaltic and either from B.C. or from the Olympic Peninsula.[24]

48°04′24″N 122°47′22″W / 48.07333°N 122.78944°W / 48.07333; -122.78944 (Fort Townsend State Park)

Four Mile Rock (also Fourmile Rock) is a round granite erratic, approximately 20 feet (6.1 m) across, in the intertidal zone below Seattle's Magnolia Bluff and 60 yards offshore.[25] It has had a navigational light placed on it and appears on nautical charts.[26]

Native Americans called the rock LE'plEpL, also written La'pub, and also called it Tele'tla (meaning "rock"). A legend says that a hero named Sta'kub could throw a giant cedar and hazel branch dragnet over the rock while standing at the beach.[27]

47°38′20″N 122°24′48″W / 47.63889°N 122.41333°W / 47.63889; -122.41333 (Four Mile Rock)[28]

15 feet (4.6 m) King Fourmile Rock, Magnolia at low tide, with yardstick.JPG
Frog Rock, a Bainbridge Island landmark

47°41′46″N 122°31′24″W / 47.69612°N 122.52347°W / 47.69612; -122.52347 (Frog Rock)

7.33 feet (2.23 m) Kitsap Bainbridge Island Frog Rock.jpg
Lake Lawrence erratic in the Puget Sound lowlands near Lake Lawrence in Thurston County. The southernmost glacier in this list, near the limit of the Yelm lobe of the Vashon Glacier in the Rainier (city) area.[29]

46°51′54″N 122°34′45″W / 46.86507°N 122.57904°W / 46.86507; -122.57904 (Lake Lawrence erratic)

15 feet (4.6 m) Thurston Lawrence Lake erratic 1.jpg
Lake Stevens Monster near Lake Stevens. 34 by 78 feet (10 m × 24 m) and 210 feet (64 m) in circumference. Largest known erratic in Washington State as of 2011,[30] and may be largest in the United States (but not North America; see the Alberta Big Rock).

47°59.816′N 122°6.954′W / 47.996933°N 122.115900°W / 47.996933; -122.115900 (Lake Stevens Monster)

34 feet (10 m) Snohomish Lake Stevens Monster 2015b.jpg
Lone Rock, the namesake landmark of the unincorporated community of Lone Rock, located on the Hood Canal tidal flat about 400 feet off shore. At least 50 feet (15 m) across.[31]

47°39′47″N 122°46′12″W / 47.66297°N 122.769916°W / 47.66297; -122.769916 (Lone Rock)

50 feet (15 m) Kitsap Lone-Rock-Hood-Canal-Washington-18-Aug-2017.jpg
Oyster Bay erratic on the shore of Oyster Bay in Thurston County

47°6′15.5″N 123°4′3.0″W / 47.104306°N 123.067500°W / 47.104306; -123.067500 (Oyster Bay erratic)

12 feet (3.7 m) Thurston Oyster Bay erratic 1.jpg
The Skystone in Bonney Lake, an andesite erratic that may have astronomical significance to the Puyallup tribe of Native Americans. It was called by an archaeastronomer "the new world Stonehenge". Height and width 4.5 by 12 feet (1.4 m × 3.7 m).

47°09′14″N 122°11′41″W / 47.153807°N 122.194685°W / 47.153807; -122.194685 (Skystone)

4.5 feet (1.4 m) Pierce Skystone.jpg
Waterman Rock in Saratoga Woods Preserve near Langley[20][32][33]

48°03′38″N 122°27′47″W / 48.06056°N 122.46306°W / 48.06056; -122.46306 (Waterman Rock)

38 feet (12 m) Island Waterman Rock east side with yardstick.JPG
Wedgwood Rock is a glacial erratic (and known to geologists as the "Wedgwood Erratic") near the neighborhood of Wedgwood in Seattle, Washington. It is 80 feet (24 m) in circumference and 19 feet (5.8 m) or 26 feet (8 m)[34] in height. Since 1970 it has been an offense punishable by a $100 fine to climb the rock.[35]

47°40′51″N 122°17′50″W / 47.68084°N 122.2973°W / 47.68084; -122.2973 (Wedgwood Rock)

26 feet (8 m) King Wedgwood Rock, yardstick.JPG
White Rock, metasandstone turned white from bird guano, in the intertidal zone at the head of Hood Canal[36][37]

47°53.593′N 122°38.579′W / 47.893217°N 122.642983°W / 47.893217; -122.642983 (White Rock)

24 feet (7.3 m) Jefferson


  1. ^ Troost & Booth 2008, p. 12 "During the period that the Vashon-age ice sheet covered the region, a tremendous volume of pressurized water was carried by subglacial streams and was responsible for carving the deep troughs of the modern Puget Sound."
  2. ^ a b Booth, Troost & Shimel 2008.
  3. ^ a b Geology of Seattle and the Puget Sound on YouTube, narrated by Nick Zentner (Central Washington University Department of Geological Sciences). Uploaded March 2, 2015 by (Nick Zentner and Tom Foster: Discover the Ice Age Floods).
  4. ^ Bretz 1913, p. 34.
  5. ^ Goldstein 1994.
  6. ^ Pailthorp, Bellamy, "Mima Mounds continue to mystify scientists", KPLU Wonders, KPLU
  7. ^ Troost & Booth 2008, p. 2.
  8. ^ Troost & Booth 2008, p. 5.
  9. ^ Martha Lake Airport Park, Snohomish County, Washington, retrieved 2015-04-05
  10. ^ David C. McConnell (March 8, 2012), History of Martha Lake Airport Community Park and "Big Rock", Snohomish County Parks Department
  11. ^ Snohomish County Parks e-Newsletter, Snohomish County, Washington, April 2012
  12. ^ "Airport Boulder". October 31, 2012.
  13. ^ Dave Tucker (April 12, 2011), Dave Tucker (ed.), "Bellingham area glacial erratics: The Arroyo Park erratic", Northwest Geology Field Trips
  14. ^ Hutton, Jane (December 2012), "Distributed Evidence: Mapping Named Erratics", in Ellsworth, Elizabeth; Kruse, Jamie (eds.), Making the Geologic Now;Responses to Material Conditions of Contemporary Life, pp. 99–103, ISBN 978-0-9882340-2-4
  15. ^ Dave Tucker (December 12, 2009), Dave Tucker (ed.), "'Big Rock' (another one) in Duvall, Washington", Northwest Geology Field Trips
  16. ^ Duvall Visitors Guide (PDF), Duvall Chamber of Commerce, 2015
  17. ^ a b Louis T. Corsaletti (March 10, 1998), "Big Rock, Big Trees, Little Park -- New Safeway Being Built Near Tiny Duvall Landmark", The Seattle Times
  18. ^ a b "Washington Rock is County Park", Reading Eagle, Reading, Pennsylvania, p. 42, May 15, 1977
  19. ^ "UW professor puts Big Rock in its geologic place", South Whidbey Record, June 25, 2008, archived from the original on June 9, 2015
  20. ^ a b Burnett, Justin (January 4, 2012), "Big Rock for sale: Giant stone goes with Coupeville apartments", Whidbey News-Times, retrieved June 1, 2015
  21. ^ Whalen, Nathan (January 27, 2012), "Coupeville Town Council unmoved by Big Rock", Whidbey News-Times, retrieved June 1, 2015
  22. ^ McShane, Dan (July 18, 2011), "Conglomerate Erratic and Link to Northwest Geology Field Trips Write Up of a Huge Erratic", Reading the Washington Landscape
  23. ^
  24. ^ Leslie Aickin (January 2008), The Geology of Fort Townsend State Park (PDF), Jefferson Land Trust Geology Group, p. 1
  25. ^ Ray 1891, p. 37.
  26. ^ Puget Sound – Shilshole Bay to Commencement Bay (PDF) (Natucial chart), National Oceanic and Atmospheric Administration National Ocean Service, Office of Coast Survey, 2015, NOAA Chart 18474, archived from the original (PDF) on 2015-04-27, retrieved 2015-05-25
  27. ^ Waterman 1922, p. 188.
  28. ^ U.S. Geological Survey Geographic Names Information System: Fourmile Rock
  29. ^ Timothy J. Walsh and Robert L. Logan (2005), Geologic Map GM-56: Geologic Map of the East Olympia 7.5-minute Quadrangle, Thurston County, Washington (PDF), Washington Department of Natural ResourcesCS1 maint: uses authors parameter (link)
  30. ^ Dave Tucker (July 18, 2011), "The Lake Stevens monster- largest erratic in Washington. Largest in the US?", Northwest Geology Field Trips
  31. ^ Measured on Google Maps overhead imagery August 24, 2017.
  32. ^ Tucker, Dave (February 11, 2010), "Whidbey Island Erratics", Northwest Geology Field Trips
  33. ^ Sheets, Bill, "Big boulder in Edmonds one of many left by long-gone glaciers", Everett Herald, archived from the original on June 21, 2015, retrieved June 1, 2015
  34. ^ Troost & Booth 2008.
  35. ^ David Wilma (July 24, 2001), Seattle Neighborhoods: Wedgwood -- Thumbnail History, HistoryLink
  36. ^ Dave Tucker (January 21, 2011), Dave Tucker (ed.), "A Really Big Erratic White Rock in Jefferson County", Northwest Geology Field Trips
  37. ^ Dave Tucker, "White Rock, a large erratic at Hood Head, Hood Canal", Northwest Geology Field Trips

External links[edit]