USGS - science for a changing world

Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK: PART Il NOTES ON THE GEOLOGY

button link to field trip map

Deer Park - Blue Mountain

STOP 6: Blue Mountain

Few viewpoints in the Olympics better allow the imagination to re-create the time of the Ice Age than does Blue Mountain. To the north, look out over the plains and waterways to see where the southern margin of the great Cordilleran ice sheet lay about 15,000 years ago.

On the way up Blue Mountain are large boulders of white granite along the roadside (at about 4.9 miles from Deer Park, or 3.0 miles from the park boundary). As far as is known, there is no bedrock of granite anywhere in the Olympics. Thus these boulders must have been brought here by the Cordilleran ice; boulders of rock types characteristic of the North Cascade Mountains and British Columbia Coast Ranges are common up to elevations of about 3,500 feet all around the north and northeast end of the Olympic Mountains. Visualize the great mass of ice building up and around the dam of the Olympics, one branch of ice flowing out along the Strait of Juan de Fuca to the sea, the other flowing south in the Puget Lowland beyond the city of Olympia, where the ice finally melted as fast as it advanced.

Development of ancient Lake Morse
Fig. FT 12. Ancient Lake Morse and diversion of Morse Creek as seen from Blue Mountai, and the outlined location of Field Trip Stop 8. click there to sneak forward.

Northwest of Blue Mountain are tree-covered flats on a broad, low divide between the ridge of Blue Mountain and Round Mountain (figs FT 12 and FT 14). The edge of the Cordilleran icecap pushed across this divide between the two mountains, for this area is covered with debris left by the icecap, and the slopes leading westward into Maiden Creek are likewise veneered with outwash from the icecap. Even from Blue Mountain rounded outcrops of lava smoothed by the scraping of ice are visible on Round Mountain. Compare the smooth shape of Round Mountain with the jagged, unglaciated cliffs of Mount Angeles.

More granite boulders are scattered throughout Morse Creek valley, its tributaries, and all the other forested valleys west of Blue Mountain, up to an elevation of about 3,500 feet. These boulders, too, might have been carried in by the icecap, although there are no rounded knobs and smooth ridges to indicate that the glacier filled the valley. But with the icecap pressed close around the mountain front, the streams draining the mountain would probably be dammed If the valley were filled with a lake at the toe of the icecap (fig FT 12,B), icebergs breaking from the ice, laden with foreign rocks and gravels, could have floated out into the lake where they slowly melted and dumped their load of debris to the bottom far from the edge of the ice. Such ice barges have indeed been observed in present-day northern latitudes where icecaps and glaciers still exist.

Morse Cr. profile
Fig. FT 13. Morse Creek profile.

The icecap and lake can explain some of the landscape seen today, but two peculiar features are not so easily explained. At the head of Morse Creek, the Cox Valley is broad and flat, singularly different from the narrow valleys of the Morse Creek tributaries. Also, Morse Creek takes an odd bend (see fieldtrip map). Flowing for several miles toward the east-northeast and eroding its valley in the relatively soft shale and sandstone, it suddenly takes a sharp swing to the north and cuts through a thick ridge of resistant lava.

Look at the low divide between Round Mountain and Blue Mountain. The divide lines up vertically as well as horizontally with the flat bottom of the Cox Valley (fig. FT 13), suggesting that the Cox Valley and the low divide are both parts of a once continuous valley.

Round Mountain and view to northFig. FT 14. View from Maiden Peak (west of Blue Mountain), looking north to Round Mountain and scene of Morse Creek diversion.

A no doubt greatly simplified history might be that before the continental ice filled the lowlands to the north, Morse Creek flowed northeast around the south side of Round Mountain. thence northward over soft sedimentary rocks to the Strait of Juan de Fuca (fig. FT 12A). Its position here was determined by a long period of erosion, during which time the more resistant lavas of the Mount Angeles-Round Mountain ridge stood out above the valleys eroded in softer rock.

When the icecap grew and advanced to block Morse Creek, a lake was formed that eventually spilled over the lava ridge just west of Round Mountain, perhaps following a course to the strait between the icecap and the mountain front (fig FT 12,B). When the icecap began to melt away as world climates warmed up, Morse Creek was trapped. It had cut a notch in the hard lava ridge and was already lower than its old course across the low divide. As the creek cut an ever-deepening gorge through the lava, the lake drained away. The land probably rose to some extent as the great weight of ice was removed from it; thus the creek cut well below its ancient course. The Cox Valley and the low divide south of Round Mountain may be remnants of the ancient Morse Creek valley.


On to Stop 7. Hurricane Ridge Road

Link to Field Trip Stop 8
| Olympic National Park geology home | | Olympic National Park | | Geologic Map | | Field Trip Map | | Geology field trip | | Site contents |

Material in this site has been adapted from Guide to the Geology of Olympic National Park by Rowland W. Tabor, of the USGS. It is published by The Northwest Interpretive Association, Seattle.

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://geomaps.wr.usgs.gov/parks/olym/ftstop6.html
Page Contact Information: Webmaster
Page Last Modified: 13-Dec-2016@16:42