1.RouteXiaweidian—Dingjiatan—Dajuesi2.Ob Servations and Purpo SeContents(1)Observe river erosion and its geomorphic consequences.(2)Observe basalt and fossils at Junzhuang village.(3)Observe the weathering of a granodiorite at Dajues.PurposeTo understand exogenic geological processes,both modern and ancient.3.Main observation pointsStop 1 Erosion and deposition in a rivers ystem,as seen in a branch oft he Yongding River,2 km Southwest of Xiaweidian VillageLocation:2km west of Xiaweidian,in the river valley on the south side of the road.Content:(1)Observe erosion and sedimentation by sheet flow and flood flow.(2)Observe erosion at the concave river bank and deposition(point bar)at the convex bank.Description:(1)Before arriving in the riverbed,observe the sediment formed by laminar flow,namely the talus on the north side of the road,below the hillside.Observe the character of the talus,for example,the nature of the deposit,distribution,geomorphic form and structure.Talus is a deposit of angular blocks in a sandy matrix,forming a fan at the bottom of the slope.The composition is simple,reflecting the bedrock on the hill above.The deposit consists of sand,silt and clay,with coarse blocks.The individual clasts are poorly rounded and poorly sorted,and the deposit is poorly stratified parallel to the slope.(2)Observe the formation oft he pluvialg ully and pluvial fan.The gully was formed by fluvial erosion.Gullies are always situated in the lowest parts of valleys,have a linear character and display“V”－shaped cross-sections.Gullies normally have steep walls and contain very little sediment in the stream bed.Any sediment present will be poorly sorted and poorly rounded sand and gravel.Most of the sediments are deposited at the mouth of the gully,where they form an alluvial fan.Alluvial fans are called fans because of their geomorphic shape.They Show a gradation in sediment size from coarse-grained,poorly sorted,subangular cobbles and gravel at the top of the fan to finer-grained sand and gravel at the base of the fan.As the gradient of the fan decreases towards its base,the sediments become thinner and discontinuous layering is developed.The composition of the sediments i S usually simple,because they were derived from the bedrock on the hillside behind the fan.(3)Observe the formation of a meander,and the nature of the geologic processes att he concave and convex river banks.1)First,observe the nature of waterf low around the bend in the river.Note thatt he water flows rapidly on the concave side of the river bend and slowly on the convex side.2)Because of the rapid water flow along the concave bank,erosion is concentrated there.The stream undercuts the bank,causing the bank to recede rapidly.Because the water flow is very slow on the convex bank,deposition occurs there,cau Sing the bank to migrate outward.Thus,the river position is constantly shifting,causing a meander.3)observe the character of the sediment,for example,the size and shape of the grains,their degree of rounding and degree of sorting.Compare these with the gully deposits observed above.4)Observe the character of the alluvium on the floodplain(flat area above the stream bed),compare it with the alluvium in the riverbed,and consider why they are different.5)Understand the processes involved in deposition of alluvium in stream valleys.Stop 2 The river terraces at Dingjia beach and YongdingheLocation:On the small massif east of Dingjia beach.Content:Observe the Pleistocene river terraces and alluviumDescription:Consult Fig.3-1.Fig.3-1 Cross-section showing the terraces at Dingjia beach and Yongdinghe(1)At this location the riverbed is about 40 m below the road,and even during floods,the river cannot rise that high.On the nearby hill,there is a gravel layer,about 2-3 m thick,that contains abundant well-rounded and moderately well-sorted clasts.This layer also has a lot of red clay,so it is red in color.Most of the clasts are 2-5 cm across,and they all consist of rocks that are resistant to weathering.The character of the gravel indicates that is a deposit of stream gravel.The sediment is dated as middle Pleistocene（～0.17Ma)by microfossils(Cheng Jie,personal communication).So,we can co nclude that the gravel layer was depo sited o n a river terrace abo ve the Yo ngding River,which suggests that the river bed was o nce at that level.The river bed is now abo ut 40 m belo w the upper terrace,indicating that the river erod ed the valley do wnward by this amo unt.(2)Paleo-river valley:Look westf rom the massif(where there is a telephone pole),where one will see a low-lying,u V”shape valley,trending E-W.The botto m o f the valley is approximately as high as the massif,o r a little higher.This valley is a paleo-river valley.Deep-cut meander:In this lo catio n o ne can see that the Yo ngding River for ms a prof ound meander curve.Bo th o f the valley sides are steep,indicating that do wnward erosion of the stream was rapid.However,the meander formed at a time(Miocene and Pleistocene)when the river was eroding laterally,rather than downward.Once the meander had fo rmed,the area was uplifted and river began to erode do wnward again.Thus,a deep-cut meander was preserved.Stop 3 Basalt and Permian sedimentary rock on the north scarp of railway Cut at JunzhuangLo cation:Beside the railora d,1 km sou th of Junzhuang.Co ntent:Observe the sedimentary sequence,cross-bedding and character of the basalt.Descriptio nThe upper Permian(P2)consists mainly of thick-bedded sandstone with interlayers of black,fine-grained sandsto ne and shale.No te the large-scale cro ss-bedding in the sandstone.At this locality the dip-direction and angle of the bedding is 135°∠30°.Closely observe the strike and dip of the main layer and the cross-beds.Also observe the change in grain size of the sandstone in a single layer from the bottom up-this is graded bedding.Note that the cross-beds are steep at the top and flatten downward-this allows you to determine that the beds are right side up,not overturned.The direction of the slope also indicates the direction in which the Water was flowing when the cross-bed was formed.Also,observe the jointing structure and a small faulti n the sandstone.At the mouth of the railroad tunnel,south of the previous stop,there is an outcrop ofe arly Jurassic basalt,which is part of the Jiulongshan Fm.(J1j).This basalt has a light red,grayish or reddish-gray color,and is cryptocrystalline and contains both vesicles and amygdules(vesicles filled with alteration minerals).Here the amygdules are filled mostly with calcite,locally accompanied by quart Z.The basaltc onsists of t hick layers,separated by beds of i gnimbrite or l enses of g ravel.The layers have a strike-dip of 140°∠32°.Several groups of joints,which cut through the basalt at different angles,are present.The main group ofjoints has a strike-dip of 347°∠68°.The basalt layer is parallel to,and overlies,the lower Permian sandstone with the omission of Upper Permian and Triassic strata to form a disconformity.Stop 4 Sill and contact metamorphism between the Ordovician Majiagou Fm.limestone and a dioriti C sillLocation:West hillside in Huiyu village.Content:Observe the folding in the sedimentary rocks,the chilled margin of the sill and the characteristics of the skarn in the country rocks.Description:(1)At this stop,limestone ofthe Ordovician Majiagou Fm.crops outi n the eastern hillside in the form of an anticline.The hinge line of the anticline strikes essentially E-W and the limbs dip north and south.(2)On the western hillside,a gray to yellow-gray vein intrudes the Ordovician limestone.Because the vein is parallel to the layering in the limestone(Fig.3-2),it is termed a sill.This vein consists chiefly of feldspar and quartz with a few dark minerals.The vein has a crystalline texture,is fine-grained and has a blocky structure.Based on the texture and composition,the rock is termed aplite.Note the 4-10 cm wide chilled border in the aplite sill where it intrudes the country rocks.In the chilled border(due to rapid cooling)the rock has a very find-grained to aphanitic texture.Note that the grain size gradually increases toward the center of the sill where it has a medium to coarsegrained texture.Note also that the limestone into whichthe sill is intruded has a baked zone,5-8 cm wide,in which the limestone has been recrystallized to marble with some pyrite crystals.Fig.3-2 Intrusive contact of a sill along the layer of Ordovician limestone in the western hillside in Huiyu villageBoth the sill and the country rock dip to the north.This outcrop is essential the north limb of the anticline observed at stop 1.Because the sill is deformed,we infer that it formed before the folding of the host rock.Stop 5 Dajueshi Formation ofg rus,spheroidal weathering and exfoliation of granodioriteLocation:Back of hill at Dajuesi.Content:Observe granodiorite,spheroidal weathering and plant-related weathering.Description:（1)Observe the color,texture,structure,main minerals and minor minerals of the granodiorite and properly identify the rock with its correct name.Overall,the rock has a pale grayishbrown color,a medium-grained,equigranular texture,and a blocky structure.The rock consists mainly of plagioclase,K-feldspar,and quart Z,with minor amounts of biotite and hornblende.(2)Spheroidal weathering(Fig.3-3):The granodiorite has three sets ofjoints that are nearly orthogonal(75°∠72°，340°∠80°，230°∠17°）and cause the rock to break into blocks.Chemical weathering along the joints is most rapid where two sets ofjoints meet,leaving a fresh zone between them that is nearly spheroidal,hence the term spheroidal weathering.(3)Plant-related weathering(Fig.3-3):Plant roots grow into cracks and crevices in the granodiorite,causing the cracks to widen.This process of physical weathering breaks the rocks into smaller pieces and allows chemical weather to proceed more rapidly.Observe the effect of different plants on the rock.Fig.3-3 Sketch diagram of spheroidal weathering and plant-related weathering ofg ranodiorite4.Brief SummaryIn this route,we have observed geological phenomena related to fluvial erosion and deposition and the development of terrace S,which show uplift of the area and downcutting by the river system.In addition,we observed and sketched contact metamorphic rocks and spheroidal weathering in granodiorite.5.Questions for discussions1)What is a river terrace and how does itf orm?2)What conditions are required for spheroidal weathering and what are the geological processes involved?
