Geologic Guide to the Deer Park Area
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fig. 1. Simplified geologic map of the Mount Angeles-Deer Park Area
(click on image for a PDF version)

Along the Deer Park Road



Soon after the traveler on the Deer Park Road leaves behind the fern and rhododendron forests and swings onto the winding roadway on the hillside above Maiden Creek (about 2.3 miles from the Park boundary), he cannot help but notice the tilted layers of rock along the roadcut. These layers are called beds by the geologist; etched in relief, they form a decorative wall along the road almost all the way to Deer Park. Stop the car at a safe turnout and examine the beds (fig. 1, geologic interest point 1).

*Marginal numbers refer to points of geologic interest on figure 1.

In many places a regular alternation of light-gray sandstone with dark-gray to black shale is conspicuous; the sandstone beds are hard and angular on the edges and project farther out than the darker, somewhat crumbly and softer shale beds. These are rocks formed from sand and mud deposited in a sea that once covered most, if not all, of the Olympic Peninsula about 60 million years ago. Most of the sand and mud was washed into the sea from the ancestral ranges of the Cascade Mountains to the northeast and from Vancouver Island to the north.

Look closely at the sandstone. On broken surfaces, the sand grains can be easily seen. The much finer grains of the black shale are less easily recognized, but if you scratch a piece of shale, the dust produced is nothing more than dried mud.


At 5.9 miles from the park boundary, where the road begins to break out into the open, thicker beds of sandstone appear. High on the mountainside they form conspicuous ribs. And, in fact, the road becomes quite rough here and just a little steeper, reflecting the greater difficulty of making a roadcut through these hard beds. A close look at the rocks reveals small rounded pebbles in the sandstone, evidence of ancient gravels.


Since layers of sediment are deposited horizontally in the sea, we must conclude that something remarkable has happened not only to tilt these beds but to raise them so far above the sea. The tilted beds here are actually tightly folded in much the same fashion as the sides of an accordion. The earth forces which produced this folding are as yet poorly understood, but their existence is in evidence in the folded rocks of almost every mountain range in the world. Along the road, we do not see the "bends" of the folds very often for they are as obscure as the actual "bend" in a tightly folded piece of paper. There is, however, indirect evidence of folding, as outlined below.

When the sediments were deposited in the ancient sea, structures were developed which, being controlled by gravity, show which side of the bed was up. For instance, when a great flood of debris washes or slides into the calm sea depths, the coarser grains and particles settle out first and the finest mud last. This sorting gives rise to graded beds, layers in which the sand grains grade from coarse at the bottom to fine at the top (fig. 2). A close look at the very thin sandstone layers between beds of shale along the road will ofttimes reveal this gradational change, sometimes expressed as a gradual change in color from light at the bottom to dark at the top. Sometimes currents along a sandy bottom tend to disrupt the original bedding and deposit irregular, sloping crossbeds that are gently curving upwards. Figure 2 shows, in an exaggerated way, how these thin curving beds look on a broken rock surface. They, too, have a definite top. Along the Deer Park Road these sedimentary features show that the rocks are folded; commonly the tops of the beds point in opposite directions.

fig. 2. Structures in sandstone beds show the presence of a fold.

On the Summit of Blue Mountain




On the south slope of Blue Mountain, knobby outcrops rise from the smooth, flowered meadows. The ancient sea was not as serene as we might have imagined; its bottom was periodically boiled by volcanic eruptions of molten rock, submarine lava called basalt. The knobs and pinnacles studding the slope are mostly composed of large and small angular fragments of basalt mixed with shale and sandstone. These rocks were once mudslides and piles of rubble which slumped off the lava flows. A lava flow is prominently displayed on the long ridge stretching northwest of the summit. The eroded edge of the upended flow stands up like a great dinosaur back (fig. 3), for the basalt of the flow is more resistant to erosion than the surrounding shale and sandstone. This variable resistance to erosion explains why the small accumulations of volcanic rubble on the south side of the mountain stand up in weird pinnacles, whereas the softer, sedimentary rocks are smoothed out between the pinnacles in slopes of fine debris.

fig 3. Upended flow of resistant lava in dinosaur-back ridge north of Blue Mountain.



The red, purple, and green colors of the rocks on Blue Mountain — especially noticeable on the summit — set them off from the gray and black rocks seen along the road below. These more colorful rocks become more conspicuous beyond the place where the road to Deer Park rounds the spectacular point that first reveals the high Olympic Peaks to the south (just before the sign announcing the fire guard station). When we look closely at the colored rocks, we find they are shale and limestones. Because the shale and limestone take on these rather uncommon colors only near the lava flows, we conclude — with the help of chemical analyses — that they are colored by iron and manganese in various chemical forms derived from the lava flows or from hot springs and steam vents associated with the volcanic eruptions. In some places the colored shale and limestone are very hard; a knife will not scratch them. These rocks have been permeated with silica in the form of quartz, one of nature's very common hard minerals (see below).

In a few red limestone layers such as the conspicuous one on the summit of Blue Mountain, one can find the globular shells of nearly microscopic animals which once lived in the sea. These fossils are distinctive, and, comparing them with similar fossils from areas where the age sequence of strata is clearly displayed, allows the geologist to assign the geologic age of Lower Eocene (about 60 million years ago) to the rocks on Blue Mountain.



Turning from the rocks at hand to the grand view, we find that the tilted lava beds on Blue Mountain are rather small and insignificant compared to the great masses of lava seen in distant peaks. To the south the rugged pinnacles of The Needles and Mount Deception rise conspicuously above the rounded ridges to either side. The cliffs of Mount Angeles (fig. 4) and Klahhane Ridge also indicate that the rock is erosion-resistant lava. For countless years water and glaciers (see below) have gnawed into the elevated bit of Earth's crust we call the Olympic Mountains and because of subtle differences in rock hardness, such as friability and solubility, have worn away the softer rocks faster than the harder ones. Liken this process to the weathering of a timberline snag; the harder fibers of the wood grain stand out in bold relief. The details of the mountains are, to a great extent, controlled by the differences in rock hardness or resistance to erosion, and to some extent the configuration of the valleys and ridges is also so controlled.

fig. 4. Panorama looking south from Blue Mountain (V means volcanic rock) (click on image for a PDF version)

Excursions from Deer Park



The meadows of Deer Park lie on a gently rolling bench perched high above the steep-sided valley of Grand Creek. Elsewhere, we see the steep valley sides rising more or less unbroken to the summits of ridges and peaks. To find out why there is a bench here, stroll down the slope south of the lower campground to one of two promontories of rock, popularly called Lookout Rocks. These promontories, high-cliffed above Grand Creek, are composed of basalt lava.

This lava is part of a thick, upended lava layer that extends from west of the Deer Park campground along this hillside to the Gray Wolf River on the east and beyond (fig. 1). Imagine the lava like a wall, once sandwiched between sandstone and shale beds; now the wall has been denuded of the easily eroded sedimentary rocks on the valley side, but it holds up the soft sedimentary rocks on the ridge side. And indeed, it seems likely that the soft sediments between this resistant wall and the volcanic rocks of Blue Mountain have been leveled by erosion into the Deer Park bench. Note where the streams on either side of the Lookout Rocks have breached the lava wall and eaten into the bench.



A close look at the bulbous surface of the Lookout Rocks will explain why the geologists call this rock pillow lava (fig. 5). Just as the other volcanic rocks of the area were erupted beneath the sea, so too the pillow lava. Naturally, no one has had a close look at hot lava erupting on the ocean floor, and students of geology still argue heatedly about how pillows form. At favorable outcrops one can see that some pillows form by a budding process. The hot lava appears to have formed a globule, that is a pillow; its surface quickly cooled and hardened as the sea water cooled the lava. But the hot lava of the pillow core was still liquid enough to ooze out from cracks in the side and form another globule. Chains of such pillows, one budding from the next, can sometimes be found. Other outcrops suggest that pillows may form singly at the erupting fissure and roll down the sides of a pile of accumulating volcanic debris to be mixed with the ocean muds as they pile on top of each other.

fig. 5. Pillow basalt south of the Deer Park campground.

Figure 5 shows that the pillows have a rounded top, that is, a humped back. If a hot, soft, young pillow rolls down on several old solid ones, the younger one tends to drape over or droop down between the humps of its elders. Thus the younger pillow is hollow on its underside and humped on its back, and we can determine which side was up, even when the lava bed is standing vertically.



East of the flowered fields of Deer Park, the Slab Camp Trail skirts the hillside and leads to fine views of meadows and mountains. In places the hillside fairly bristles with splinters of slate. To find out how these splinters form we must go back to the time when the rocks were deeply buried and being folded.

Shale forms from mud buried under a great load of overlying sediment, and we find that as the pressure increases during folding of the shale beds, the rock may become even harder and will eventually be transformed into slate. But slate is not only hard, it is thinly sliced by smooth fractures (producing the thin tablets once used in schools). Figure 6 shows one way these fractures develop during the folding of a shale under great pressure. The original shale layer is usually made up of many very thin beds and will have a tendency to break along these beds. The intersection of the closely spaced slate fractures and the original bedding combine to chop the rock into the splinters (fig. 6). These splintered slates are most common near thick lava beds which are little shattered by the forces of folding compared to the sedimentary rocks. Thus it seems probable that the lavas resisted the deformation and in doing so subjected the nearby sediments to even greater pressures as they were squeezed against the rigid lava.

fig. 6. Development of slate splinters. (Simplified: spacing of fractures and beds not to scale)


A hike along the trail from Deer Park to Obstruction Peak is a "must" on a visit to Deer Park. Beyond the pine-covered south slopes of Green Mountain are ridge-top meadows, fir thickets, and green vales. Battalions of yellow glacier lilies crowd around the retreating snowdrifts where the trail stretches off to small but attractive Maiden Lake. But even before the trail reaches the Elysian fields, some interesting rocks can be found.


Where the trail crosses the first subsidiary ridge of Green Mountain is a large outcropping of gray sandstone. Fragments of this sandstone commonly have at least one smooth flat surface where the rock has broken along a bed, and on such surfaces we can see the sparkle of small flakes of white muscovite, a mineral of the mica group commonly known as isinglass. Sixty to 70 million years ago this same mica sparkled in the Eocene sun on rocky peaks of the ancestral Cascades. Along with quartz and other minerals it was washed down to the sea by streams and carried into the sea bottom by currents. Pressure and chemical changes transformed the mica-bearing sands into sandstone and muds into slate. In the Olympic Mountains the rocks have been uplifted and the mica exposed once again by erosion to sparkle in the sun.



Look closely at the outcrops or at blocks of sandstone along the trail as it climbs along the south side of Green Mountain; here and there in the sandstone are cracks filled with white quartz crystals. In some places one can find a specimen in which the tiny crystals are well formed and glassy clear (fig. 7).

fig. 7. Quartz crystals (very well farmed and greatly enlarged)

As the sandstones themselves are rich in quartz, we surmise that these crystals formed in the once-buried rock from solutions that dissolved minute bits of silica from the sand grains and redeposited it as small crystals in open cracks. More such crystals can be found in thick sandstone beds along the trail west of Maid en Lake.

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Last Updated: 20-Aug-2010