WALLOWA GEOLOGY An illustrated rock-hunters guide to Hurricane Creek By Ellen Morris Bishop From youngest to oldest, they are: For The Chieftain A utumn, when streams are low, is an ideal time to go looking for geology. Creeks and rivers carry samples of every kind of rock in the Wal- lowas down to us. Tumbled and rounded, only the hardy ones sur- vive. But in the dry portions of stream beds, you can ind virtual- ly every kind of rock in the Wal- lowa Mountains. Each individual stone tells a story. Some bear witness to monstrous ancient volcanic ex- plosions. Others tell of languid seas and vanished life. There are faults in some rocks. Ancient loods in others. And in each rounded river rock, there is the haunting memory of time. You hold in your hand not just a rock, but a stream, a beach, a torrid lava low, all from so long ago that all human memory fails. Fortunately, geologists and rock-hunters are good detectives and good story-tellers. This guide will help you determine what kind of rock you have found, and what story it tells. What are the Wallowa Moun- tains made of? Five different types of rock comprise the Wallowa Moun- tains. Examples of all of them can be found in Hurricane Creek and in the ephemeral streams and glacial gravels that oc- cupy most of the valley loor. The photos below are all rocks from Hurricane Creek. Each is unique. Each tells a story. The rocks that you will ind will be some- what different, but will belong to one of these rock types. Happy hunting! COLUMBIA RIVER BASALTS: (Igneous, volcanic) About 14-16 million years old. Youngest rocks in Wallowas. Erupted from vents (issures, or “dikes”) now exposed in the Wallowa Mountains. These lavas inundated the Columbia Basin. (They are 3 MILES thick beneath Hanford.) Some lowed all the way to the Oregon Coast. Today, they are exposed at Haystack Rock, Point Lookout, Depot Bay and many other places. Along Hurricane Creek, Columbia River basalts occupy the top of Hurricane Divide and form Twin Peaks and Sawtooth. Basalt is not abundant in Hurricane Creek, and much of what you ind has come from the basalt dikes. ID: Dark red-brown to reddish on weathered surface. Black on fresh surface. May contain holes (gas vesicles). ‘GRANITES’ (Actually granodiorite and tonalite) (Igneous, plutonic/intrusive) About 100-130 million years old. Light-colored rocks with “salt and pepper” appearance. Intruded into the older rocks of the Wallowas. The “granites” form the core of the Wallowas — the Lakes Basin, for example. Formed by melting of the lower crust and of the older greenstones and sedimentary rocks. ID: Light-colored, salt/pepper appearance. Sparkly dark minerals. Proportion of light and dark minerals will vary from one rock to another. SANDSTONE AND SHALE: (sedimentary; slightly metamorphosed.) 180-210 million years old. When found as rocks in or along Hurricane Creek, these are usually quite hard and have a “banded” appearance. They are often dark gray and/or tan. They form the tops of Chief Joseph Mountain and Hurwall Divide. They were intruded by hot granite magmas, which “cooked” them, fusing the sand-grains and clays and producing a harder rock than the original sand- stones and shales. LIMESTONES: (AKA, Marble) (sedimentary, but metamorphosed to a soft marble in many cases.) 220-230 million years old. These rocks formed as parts of reefs or extensive banks of carbonate sand (Think Bahamas Banks). They may have a banded or “marbled” appearance, but often are simply a soft, rounded gray or white rock. The enlarged crystals of calcite that form the rock may provide a “sparkly” appearance. Some mar- bles may contain recognizable fossils. Because these are soft rocks, they tend to be well-rounded and relatively small. VOLCANIC GREENSTONES: (Igneous, volcanic rocks that have been metamorphosed.) 230-270 million years old. Formed as lava lows and related processes on volcanic islands far of the Idaho coast a long, long time ago. Metamor- phosed during collision with North America about 120 million years ago and also by heat of Wallowa granite intrusions. Today these rocks are the foundation of the Wallowas and are very prevalent in Hells Canyon. Dark gray-green, or gray with a greenish caste. Along Hurricane Creek, many greenstones were initially formed as volcanic debris lows, explosive ash eruptions, and stream sediments on the ancient volcanic peaks. Others were actual lava lows. Their appearance is highly variable, but they all have a characteristic yellow-green to gray-green color. May include lighter yel- low-green veins, blobs or stripes. Wallowa.com | 27