A SELF-GUIDED TOUR FIS H RA NCH Hwy. 24 on-ramps TO WALNUT CREEK reddened by baking; these bands are called “bake zones.” OF ROUND TOP VOLCANOES RO AD AND OAKLAND IN ORDER TO REDUCE OR PREVENT THE RISK MEMBERSHIPS A related process occurs when a brushfire reddens rocks OF SERIOUS HEAD INJURY OR DEATH, STATE The Regional Parks Foundation Fish Ranch R LAW REQUIRESTHAT ALL BICYCLISTS UNDER offers memberships to park users. and soil. To see Post 6, walk down the road about 100 feet 1.08 to AGE 18 WEAR AN APPROVED HELMET WHILE Benefits include free parking, past Post 5, then bear to the right. oa By Stephen W. Edwards RIDING ON TRAILS AND ROADWAYS. THE swimming, and off-season camping d TT Before you is a wall with basalt on the left and Orinda Director, Regional Parks Botanic Garden DISTRICT ALSO STRONGLY RECOMMENDS discounts, depending on member- 6 O THAT ALL EQUESTRIANS AND ALL BICYCLISTS ship category. For more information, SK T mudstones on the right. The bedding in the mudstones is EC an o F EL WEAR HELMETS AT ALL TIMES. call (510) 544-2203. YL LD Robert Sibley Volcanic Regional Preserve features a complex d ish INE disrupted, giving the appearance of drag-folding resulting N H R N LAND BANK AREA A w a 1500 volcanic center that was the source, 10 million years ago, of TU y. n CLOSED TO PUBLIC from relative uplift of the lava occurring during the past 10 C NA TI BA 24 ch (EXCEPT ALONG SKYLINE TRAIL) 9 most of the lavas that underlie the ridges from Inspiration YF Ro CLOSED TO PUBLIC 8 Quarry Pit million years. Alternatively, the disruption of the mudstones ad 7 O D ORE Point in Tilden Regional Park to Moraga. Round Top, one of 1100 may have occurred earlier, at the time of volcanic activity. NA .12 RIV GR CLOSED TO PUBLIC ST 10 L the highest peaks in the Berkeley Hills, consists of lavas, VO E .15 LC 1200 This site was close to, or was in, the wall of the caldera, and 11 00 View of Mt. Diablo IZ AN 00 breccias (unsorted mixtures of fine and coarse volcanic deris) ZLY .11 TUNNEL would have been subject to all kinds of slumping, sliding and O LD 1300 IC .28 and tuffs (lithified volcanic ash—ash that has become stone) R OAD 1400 plowing. The “exotic” large sandstone blocks lying about that once filled a volcano. .2 .04 .27 were torn from very ancient Cretaceous (Age of Dinosaurs) 9 RE PE SK CR 1500 Though Round Top was once the infilling of a great caul- .07 AK rocks by the ascending lavas. The quarrymen took the lava, YL EA .03 TI 6 IN 11 0 dron, it stands out today because it was originally surrounded 0 ON but left the sandstone here. To see Post 7, proceed northwest TR E 1200 AI 1300 by “incompetent” (easily eroded) sedimentary rocks of the 1300 TRAIL 5 along the main road, then take the first right. L Orinda Formation, which have eroded away. During the past 1600 .1 BL LAND BANK AREA Massive basalt was removed from this major quarry BLV 7 3 VD 0 140 CLOSED TO PUBLIC .87 LAND BANK . 10 million years the Berkeley Hills were uplifted on a gigantic pit. The north wall shows a set of thick lava flows. They D. (EXCEPT ALONG SKYLINE TRAIL) 0 11 0 Quarry Pit scale because of strains on the Hayward and Moraga fault 140 0 are tilted on edge, nearly vertical, with a bake zone at the .11 systems. This uplift folded the rock formations, and the Round base of the sequence (far left). The well-defined layers near .04 TOP 3 Top vent complex was tilted on its side. Hence, folding and LEGEND .4 1 ND 15 00 2 4 the top of the face are jointing-units resulting from cooling. GRIZ Z LY OU LOOP erosion have exposed a cross section of a great volcano, right Hikers, Horses & Bicycles PE Another interesting outcrop is situated on the remnant of an R AK 00 down to its roots, providing an unsurpassed outdoor laboratory Hikers and Horses 00 16 old road along the southeast flank of this peak. There you can .63 12 1 .17 for the study of volcanism in the Central Coast ranges. Skyline National Trail/ Round Top see a thick accumulation of cemented red-baked volcanic TRAIL The blocks of stone scattered everywhere around the Bay Area Ridge Trail 1763 cinders. These kinds of deposits always indicate proximity 0 1500 0 13 (Hikers and Horses) 17 flanks of Round Top are basalt lava (a hard, dense, dark Water Tank of a vent. Loose deposits of comparable material are quarried 00 BAY .02 volcanic rock). Lava within the vent has been dated at UC Mileage Between Points BLV at volcanic vents in Lake County, and the products end up as 1600 1400 Berkeley, by the potassium-argon radioisotope dating method, KY .24 S Stream or Creek red-cinder mulch on gardens in the Bay Area. D. .15 .35 .18 LI AR at 9.5 million years before present. N .22 Parking This is a red-baked sequence of air-fall tuffs. At the base 8 EA N .08 E .15 A great diversity of volcanic phenomena is preserved for Restroom is a lava flow. Within the section are buff-colored thin .36 .03 .03 DR. study at Sibley. Basaltic dikes (feeders of the vents), tuff- W Water 2.94 miles from .08 bands that look like lava. They are really air-fall tuffs that B LV D . Round Top Loop breccias (ash containing a scattered jumble of blocks and Visitor Center W Trail southeast to R were, for some reason, more strongly lithified than the other chunks of lava), lava flows, red-baked cinder piles, air-fall EL Skyline Gate in ID VE Redwood tuffs in the section. Volcanic ash, which is called tuff when GE Gate ALE RT tuffs and the major vent itself can all be seen first-hand in the ON Regional Park 13 00 lithified, generally consists of rock fragments, crystals and Pond ND course of an easy hike. Numbered posts, which correspond HUCKLEBERRY bits of glass ejected from a volcano. Most of the tuffs at Sibley BOTANIC OR to the numbered descriptions below, have been placed at some Scale 00 0 1/4 Mile OAKLAND REGIONAL have very little glass and would thus be called “lithic-crystal” 12 DR TH of the most interesting outcrops. . PRESERVE Contour interval 20 feet tuffs. The thin, hard, buff-colored bands in this section are TR To visit this site, walk up the paved road to the EBMUD 1 Mileages courtesy Hal MacDonald, 110 0 “vitric-lithic-crystal” because they contain glass shards. AI EBRPD volunteer water tank. A dark basalt dike, an important feeder of This is a beautiful basalt lava flow. It is smooth and L lava to the caldera, cuts through a sequence of tuff-breccias 9 massive in its upper part, to the left. The lower part is (brown) and pebbly mudstones (gray), all inside and near the These tuff-breccias originated when fluid mixtures of ash, Miocene hill or peak. This unit also curves eastward through the vesicular and “jumbled.” The lower parts of lava flows are bottom of the caldera. The mudstones indicate ponding of 3 lapilli (small pieces of lava, and pebbles derived from the bay-laurel grove in the gully to the right. The steep wall across often jumbled by turbulence and drag. The vesicles in this water; the tuff-breccias are the remains of landslides and Orinda gravels) and blocks of lava, perhaps saturated by rain, the pit consists of lava that formed the basalt dome. Eventually flow were caused by escaping gas. The zeolites, chalcedony, blockfalls into the pit from the surrounding walls. slurried and rumbled down the inside slopes of the volcano. In the Round Top vent completely buried itself in basalt flows. opal and calcite filling many of the vesicles were precipitated Note Orinda Formation river sands and gravels on the 2 left against basalt lava on the right. This is part of the this outcrop there are two grades of breccia, the one at the bottom and the one to the right, bearing a much higher proportion of From this point, note the view of Mt. Diablo, which, though it contains some submarine volcanic rocks, is not, and never was, during and after cooling of the lava. Alternatively, the outcrop can be interpreted as two separate lava flows. wall of the caldera. As you walked along the road to this broken-up chunks of lava. a volcano. HOW MANY VOLCANOES? Round Top was the big one. point, you passed the steep cliffs below the summit installation. This quarry pit was made by quarry operations in which huge This roadcut exposes Orinda Formation river gravels, There are smaller ones outside the Preserve on private property These cliffs expose mostly colluvium (recently accumulated 4 amounts of massive basalt lava were removed. The result is 5 sands and mudstones. A cross-cutting basalt dike at the left to the north and southeast. Another, of rhyolitic composition slope debris), but there are also numerous outcrops of a tremendous boon to geology, for the pit exposes the interior of end feeds into the overlying volcanics. The red streaks and layers like the ash from Mount St. Helens, underlies the Lawrence cataclastic basalt flows that look much like the colluvium. the Round Top volcano. You are standing on bedded tuff-breccias, in these river beds were caused by oxidation of iron in the Berkeley Laboratory and Little Grizzly Peak in Tilden Regional Cataclastic means the flows were shattered into coarse blocks which filled much of the caldera, settling at times into a small sediments. Such varicolored “redbeds” are explored worldwide Park. About 9 million years ago it was erupting beside Round by violent motions within the caldera — perhaps by collapsing lake. To the left (north end of the pit) the layers of tuff-breccia for the fossils of plants and animals they contain. Elsewhere in Top. Subsequently it was shifted about three and one-half into voids created by eruptions of later lavas. curve around the pit, giving a hint of the inner parts of the volcano the Preserve, bands of more intense red are found at the tops and miles northwest by movement along Wildcat Fault. That walls that originally stood above the surrounding terrain as a bottoms of lava flows. In such cases iron was oxidized and makes a total of four volcanoes.
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