The Mai Maikden sedimentary sequence: a reference point for the environmental evolution of the Highlands of Northern Ethiopia

This paper presents a geomorphologic–stratigraphic analysis of a travertine dammed lacustrine–swampy sedimentary sequence, composed of clay, peaty layers and phytoclastic travertine sands, deeply incised by the Mai Maikden river, on the Highlands of Tigray (Northern Ethiopia). Radiocarbon datings of peaty deposits allow us to establish that the travertine dams have developed at least between 7310±90 yr B.P. and 5160±80 yr B.P. In the upper part of the sequence, palaeochannels filled with travertine and limestone coarse gravels and blocks, indicate the activation of concentrated erosion on the surrounding slopes. Later, both the travertine dam and the lacustrine–swampy deposits were buried by alluvial and colluvial sediments, still mixed with large amounts of organic matter, testifying to generalized slope erosion processes. The end of travertine deposition may be connected with a progressive reduction of vegetation cover and the subsequent decrease of CO₂ in groundwater. The occurrence of the latter phenomena may be related to the onset of drier climatic conditions even though the finding of a large prehistoric settlement in the area, also indicates some influence of human impact.     

Soils and geomorphic evolution of bedrock facets on a tectonically active mountain front, western Sangre de Cristo Mountains, New Mexico

Soil profiles, colluvial stratigraphy, and detailed hillslope morphology are key elements used for geomorphic interpretations of the form and long-term evolution of triangular facets on a 1200 m high, tectonically active mountain front. The facets are developed on Precambrian gneisses and Tertiary volcanic and plutonic rocks along a complexly segmented, active normal-fault zone in the Rio Grande rift of northern New Mexico. The detailed morphologies of 20− to 350 m high facets are defined by statistical and time-series analyses of 40 field transects that were keyed to observations of colluvium, bedrock, microtopography, and vegetation. The undissected parts of most facets are transport-limited hillslopes mantled with varying thicknesses (0.1 to > 1 m thick) of sand and gravel colluvium between generally sparse (≤10–30%) bedrock outcrops. Facet soils range from (a) thin (≤ 0.2 m) weakly developed soils with cumulic silty A or transitional A/B epipedons above Cox horizons in bedrock or colluvium, to (b) deep (≥0.5–1 m) moderately to strongly developed profiles containing thick cambic (Bw) and/or argillic (Bt) horizons that commonly extend into highly weathered saprolitic bedrock. The presence of strongly weathered profiles and thick colluvium suggests that rates of colluvial transport and hillslope erosion are less than or equal to rates of soil development over at least a large part of the Holocene.The catenary variation of soils and colluvium on selected facet transects indicate that the degree of soil development generally increases and the thickness of colluvium decreases upslope on most facets. This overall pattern is commonly disrupted on large facet hillslopes by irregular secondary soil variations linked to intermediate-scale (20–60 + m long) concave slope elements. These features are interpreted to reflect discontinuous transport and erosion of colluvium down-slope below bedrock outcrops. The degree of weathering in subsurface bedrock commonly increases more systematically upslope on most facets than colluvial soils. This pattern is consistent with an increase in age with height on these fault-generated facet hillslopes.The characteristic range of internal variation in soils and colluvial deposits on a given facet also varies greatly among facets with differing overall morphologies and external environments. Deep cumulic soils and thick colluvium occur consistently on steep (≥ 30°), high, and relatively undissected facets above the narrow central sections of fault segments. Much thinner and less weathered colluvium and soils overlie saprolitic bedrock at shallow depths on low, highly dissected, gently sloping (≤ 20°) facets above complex fault segment boundaries. Parametric and nonparametric analyses of variance indicate that these large-scale contrasts in facet morphology correlate primarily with a few facet subgroups related, in decreasing importance, to variations in range-front faulting, bedrock lithology, and piedmont dissection or aggradation. These factors are related to facet morphology, drainage evolution, and hillslope-soil stratigraphy in a general geomorphic model for fault-generated facets. In this model, segmentation-related changes in the geometry and/or rates of faulting most strongly affect facet size, slope gradient, the thickness of colluvium and soil development, and drainage patterns. Facets of varying heights have similar hillslope forms at the same position on the range front; these characteristic morphologies are established under prevailing tectonic and nontectonic conditions on facets as bedrock is initially exposed from beneath alluvial-covered fault scarps above a height threshold of 15–35 m.     

湖南津市黄牯山第四纪泥砾混杂堆积成因探讨

湖南省津市黄牯山发育一套泥砾混杂堆积,前人曾认为属冰川成因。近年笔者对该套堆积的沉积层序、结构组成、横向变化、砾石组构与表面特征等作了详细考察,发现堆积中下细上粗的层序特征,砾石扁平面的优选定向,沿层理经差异风化和磨蚀所形成的假"冰川擦痕",由壳状裂口经磨蚀、圆化所形成的凹面石,沉积体短距离内的相变,以及周边仅有低矮丘陵的地貌背景等,表明泥砾混杂堆积为泥石流产物,而不是冰川成因。     

Supergene and exotic Cu mineralization occur during periods of landscape stability in the Centinela Mining District,Atacama Desert

The Centinela Mining District (CMD), Atacama Desert (northern Chile), includes several mid‐late Eocene porphyry Cu deposits that contains supergene mineralization and provides access to a record of gravel deposits that host syn‐sedimentary exotic Cu mineralized bodies. By studying these gravels, we reconstruct the unroofing history and constrain the geomorphological conditions that produced supergene and exotic Cu mineralization. We present an integrated study based on stratigraphic and sedimentological data, lithology clast counts, ⁴⁰Ar/³⁹Ar and U/Pb ages from interbedded tuff layers and U/Pb detrital zircon geochronology data. To relate the gravel deposition episodes to the timing of the supergene mineralization, we provide in‐situ and exotic supergene mineral ages (40Ar/39Ar and K‐Ar). Six gravel units were deposited between the mid‐Eocene and the mid‐Miocene. The Esperanza gravels were deposited concurrently with the emplacement of porphyry Cu deposits at depth. The subsequent Tesoro I, II and III and Atravesado gravels register the unroofing of these deposits, from the advanced argillic zone to the sericitic and prophylitic hypogene zones. The Arrieros gravels register landscape pediplanation, that is, denudational removal and wear of the landscape to base level on a relatively stable tectonic regime, occurring roughly contemporaneous with supergene activity. The supergene mineral ages of the CMD define a time span (ca. 25–12 Ma) during which most of the supergene ages cluster in northern Chile. This time span corresponds with a period of warm and humid climate conditions in the southern hemisphere. We conclude that landscape pediplanation favours supergene mineralization and helps preserve the former supergene mineralized zones from significant erosion. Low erosion rates during pediplanation may constitute a necessary condition for the efficiency of the supergene processes in such semi‐arid climate.     

基于数字图像的中国西北地区戈壁表面砾石形貌特征研究

中国戈壁面积约66.08万km²,超过了流动沙丘和半固定沙丘的面积之和,但目前对戈壁沉积特征的研究程度相对较低。本文采用ImageJ软件,对中国西北地区戈壁原位无干扰的表面数字图像进行量算,获取了砾石覆盖度、粒径、磨圆度和形状比率等形貌参数。结果表明：中国西北地区戈壁表面的砾石覆盖度介于31.5%～84.6%,以中覆盖度为主,70%的戈壁属于空气动力学稳定表面;90%以上的戈壁表面砾石平均粒径为细砾和中砾。不同区域戈壁表面砾石磨圆度的平均值介于0.50～0.76,形状比率变化范围在1.38～2.46。戈壁表面砾石形貌特征与其成因类型密切相关：以剥蚀（侵蚀）-洪积作用为主形成的戈壁,砾石粒径较粗、形态比率较大、磨圆度低、覆盖度较高;以冲洪积为主形成的戈壁,砾石粒径和形态比率变小,磨圆度变好而覆盖度降低。砾石形貌特征可为追溯戈壁物源区和反演沉积物的搬运堆积过程提供参考。     

Distribution and Significance of Glacially Buried Organic Matter in Michigan's Southern Peninsula

Within Michigan's Southern Peninsula, the presence of glacially buried Pleistocene organic matter is relatively uncommon but more abundant than previously recognized. These deposits tend to be concentrated in 13 districts and are most closely related to (1) places with high relief on the drift-covered bedrock surface, (2) the extent of buried bedrock valleys, and (3) low areas on a palimpsest paleosurface. Well logs show that multiple organic horizons at any one site are extremely rare, but variations in location, elevation, types of material with their climatic implications, and radiocarbon dates support the conclusion that remnants of several Pleistocene paleosurfaces, preserved through burial by deposits from separate glacial advances, exist at a number of places, especially in the southern part of the peninsula.     

秦岭太白山北麓砾石层的成因及第四纪古冰川问题*

以直接发源于太白山北麓的几套砾石层为研究对象,从野外沉积特征和沉积物理化分析入手,对砾石层的成因作出解释,并为恢复该区第四纪古气候的演变过程及中国东部古冰川研究提供了依据。     

The structural and sedimentological setting of the Quaternary deposits in Al-Ain area, UAE

Quaternary sediments represent the main constituent which covers an ENE–WSW elongated depression some 25 km long and 10 km wide (Al-Ain area). This depression is encountered between two north and south low fault scarps and is located perpendicular to the Al Jaww plain and Jabel Hafit axes.Four main types of Quaternary surface deposits were identified belonging to: flood plain and braided channels; desert plain; aeolian sand; and sabkha. The first type shows many pedogenic and non-pedogenic features of which are dolocrete, calcrete and gypcrete.An ENE–WSW closely-spaced dip-slip, stepping pattern fault set could be traced in dolocretized-calcretized braided channel deposits, on the south margin of this depression, where the relationship between fault geometry, displacement and geomorphology suggest a model of either graben or half-graben. The role of this system in developing the landscape of Al-Ain is well documented near the surface but a comprehensive study to assess its role in the sub-surface is needed.     

Climate controls on late Pleistocene landscape evolution of the Okhombe valley, KwaZulu-Natal, South Africa

Hillslopes in central and western parts of KwaZulu-Natal, South Africa are often mantled by colluvial sediments of the Masotcheni Formation. These sediments have accreted in response to several cycles of deposition, pedogenesis and incomplete erosion. Climatic controls on these cycles are incompletely known. Results from fieldwork, micromorphology, stable carbon isotope analysis and Optically Stimulated Luminescence dating of Masotcheni Formation sediments from Okhombe valley in the Drakensberg foothills are combined. Deposition in the area had at least 11 phases, starting before 42 ka and ending before 0.17 ka. The first six deposits (from before 42 ka to after 29 ka) resulted from the interplay between slope processes and fluvial redistribution under cold conditions. Solifluction was the most important slope process. No deposits have been found from the Last Glacial Maximum, arguably because this period was too dry. The last five deposits (from about 11 ka to before 0.17 ka) resulted from fluvial redistribution of upslope material and older deposits under increasing precipitation. Current extreme gully erosion in the Masotcheni Formation indicates a lack of available upslope material, leaving downslope deposits as the only sediment source for fluvial redistribution. This model for landscape response to climate change may be able to explain how climate controlled landscape processes in other Masotcheni Formation sites in KwaZulu-Natal. In the research area and elsewhere, this proposition may be tested with numerical landscape evolution models.     

LiDAR-derived DEM evaluation of deep-seated landslides in a steep and rocky region of Japan

In steep and rocky terrains, their rough surfaces make it difficult to create landslide inventories even with detailed maps/images produced from airborne LiDAR data. To provide objective clues in locating deep-seated landslides, the surface textures of a 5 km² steepland area in Japan was investigated using the eigenvalue ratio and slope filters calculated from a very high resolution LiDAR-derived DEM. The range of filter values was determined for each of a number of surface features mapped in the field and these included: cracked bedrock outcrops, coarse colluvial deposits, gently undulating surfaces, and smooth surfaces. Recently active slides commonly contained patches of ground in which deposition and erosion occurred together near the erosion front, or where cracked bedrock outcrops and coarse colluvial deposits coexisted under a gently undulating surface. The characteristic eigenvalue and slope filter values representing this sliding process were applied to maps of the DEM derived filter values to extract potential sites of recent landslide activity. In addition, the relationships between the filter values of deep-seated landslides at various stages of evolution within the field mapped area were extended to the entire study area, to assess the contribution that landslide evolution makes to change in the landscape as a whole. While landslide components made up the steepest as well as the gentlest parts of the landscape depending on their evolutionary stage, landslides were constantly coarsened and steepened by progressive erosion, probably initiated by river bank erosion at the foot of slopes.     

Bankfull width and morphological units in an alpine stream of the dolomites (Northern Italy)

This research examines variations in bankfull cross-sections along a steep stream of the Dolomites (Cordevole stream, Belluno, Northern Italy). Field measurements were conducted to determine variations in the channel top-width at bankfull stage in relation to the drainage area and to the length of the flow path. After grouping the bed morphologies according to the Montgomery and Buffington [Montgomery, D.R., Buffington, J.M., 1997. Channel-reach morphology in mountain drainage basins. Geol. Soc. of Am. Bull. 109 (5) 596–611.] classification, we analyzed the increase in bankfull width for the dominant stream units (cascades, step pools, isolated pools and colluvial reaches at the head of the basin). We observed that the morphologies more related to the drainage area are colluvial reaches and pools; the less adaptable are steps and cascades. These differences likely result from the absence of lateral constriction in the colluvial reaches and pools, whereas the presence of coarser sediments in the bed can affect the transverse adjustments in steps and reaches dominated by cascades. Linkages between cross-section geometry and parameters related to flow (i.e. drainage area and stream power) have been analyzed together with the distribution of surface grain sizes and its coarsening pattern. The existence of distinctive bankfull widths between different morphological units points out the degree of susceptibility to be modeled according to the channel slope, reference diameter (D₉₀), and contributing area.     

Floodplain development based on selective preservation of sediments, Squamish River, British Columbia

In the conventional model of floodplain sediment accumulation, mechanisms of floodplain growth are differentiated into lateral and vertical accretion processes, in which within-channel deposits are capped by overbank deposits. In the high-energy, gravel-based Squamish River, sediments laid down on bar surfaces are composed of trough and planar crossbedded coarse sands. These sequences contrast incongruously with adjacent floodplain deposits which are composed in large part of vertically accreted fine sands atop coarse alluvial gravels. Using element analysis it is inferred that bar platform sediments are stripped away by chute channels, which are subsequently infilled with lower-energy deposits. From this, a model of floodplain growth based on selective preservation of bar platform sands and prefrential preservation of vertically accreted deposits is proposed. This mechanism of sediment replacement occurs independent of channel planform type.     

CHANGES IN DEPOSITION ON A COLLUVIAL FAN DURING THE UPPER HOLOCENE IN THE TINDASTÓLL MOUNTAIN, SKAGAFJÖRÐUR DISTRICT, NORTH ICELAND: PRELIMINARY RESULTS

This study investigates variability in depositional processes and landforms on the Innstaland colluvial fan, northern Iceland, over the late Holocene. This is completed using geomorphological mapping, historical records and relative‐age dating tools (vegetation survey, rock hardness and tephrochronology). Debris flows are the main contributors to fan development. Six main phases of deposition are distinguished, varying in deposit magnitude along a general trend of decreasing magnitude over time. With the help of dated tephra layers, the aggradation rates of the fan are calculated for the upper Holocene and in particular over the historical period (post‐ AD 1104). An episodic sediment transfer system is reconstructed, with several phases of low activity between more rapid aggradational episodes. The interpretation of the colluvial deposits suggests that the main phase of aggradation on the fan ended during the historical period after which incision started (between AD 1300 and 1766). Incision tracks act as the main pathways for debris distribution at the present time. These variations are related to the changing response of the catchment system over time.     

Integrating soils and geomorphology in mountains—an example from the Front Range of Colorado

Soil distribution in high mountains reflects the impact of several soil-forming factors. Soil geomorphologists use key pedological properties to estimate ages of Quaternary deposits of various depositional environments, estimate long-term stability and instability of landscapes, and make inferences on past climatic change. Once the influence of the soil-forming factors is known, soils can be used to help interpret some aspects of landscape evolution that otherwise might go undetected.The Front Range of Colorado rises from the plains of the Colorado Piedmont at about 1700 m past a widespread, dissected Tertiary erosion surface between 2300 and 2800 m up to an alpine Continental Divide at 3600 to over 4000 m. Pleistocene valley glaciers reached the western edge of the erosion surface. Parent rocks are broadly uniform (granitic and gneissic). Climate varies from 46 cm mean annual precipitation (MAP) and 11 °C mean annual temperature (MAT) in the plains to 102 cm and −4 °C, respectively, near the range crest. Vegetation follows climate with grassland in the plains, forest in the mountains, and tundra above 3450 m. Soils reflect the bioclimatic transect from plains to divide: A/Bw or Bt/Bk or K (grassland) to A/E/Bw or Bt/C (forest) to A/Bw/C (tundra). Corresponding soil pH values decrease from 8 to less than 5 with increasing elevation. The pedogenic clay minerals dominant in each major vegetation zone are: smectite (grassland), vermiculite (forest), and 1.0–1.8 nm mixed-layer clays (tundra). Within the lower forested zone, the topographic factor (aspect) results in more leached, colder soils, with relatively thin O horizons, well-expressed E horizons and Bt horizons (Alfisols) on N-facing slopes, whereas soils with thicker A horizons, less developed or no E horizons, and Bw or Bt horizons (Mollisols) are more common on S-facing slopes. The topographic factor in the tundra results in soil patterns as a consequence of wind-redistributed snow and the amount of time it lingers on the landscape. An important parent material factor is airborne dust, which results in fine-grained surface horizons and, if infiltrated, contributes to clay accumulation in some Bt horizons. The time factor is evaluated by soil chronosequence studies of Quaternary deposits in tundra, upper forest, and plains grassland. Few soils in the study area are >10,000 years old in the tundra, >100,000 years old in the forest, and >2 million years old in the grassland. Stages of granite weathering vary with distance from the Continental Divide and the best developed is grus near the sedimentary/granitic rock contact just west of the mountain front. Grus takes a minimum of 100,000 years to form.Some of the relations indicated by the soil map patterns are: (1) parts of the erosion surface have been stable for 100,000 years or more; (2) development of grus near the mountain front could be due in part to pre-Pennsylvanian weathering; (3) a few soil properties reflect Quaternary paleoclimate; and (4) a correlation between soil development in the canyons and stream incision rates.     

Paleoclimatic events of the Late Holocene in floodplain deposits of small rivers of Southeastern Transbaikalia

Alluvial, aeolian and pedogenic horizons were detected in floodplain deposits of small rivers of Southeastern Transbaikalia. The occurrence of pedogenic horizons in floodplain deposits indicates that they were formed at periods of an enhancement in erosion activity at the end of the dry cold phase and at the beginning of the dry warm phase during the overall course of heat availability and humidification of climate in the latter half of the Holocene. An enhancement in fluvial activity corresponded to the cold humid phase, whereas an intensification of the aeolian process was taking place during the dry cold phase.     

Revisiting William Morris Davis and Walther Penck to Propose a General Model of Slope ‘Evolution’ in Deserts

Based on research from slopes on rhyolite domes of known age formed over a million‐year continuum in eastern California, a classic geomorphic debate is reconsidered and a general model of desert slope development proposed. This study examines steep (～25° to ～35°) boulder‐dominated slopes that include well, varnished, vertically oriented colluvial deposits. Such deposits are common throughout the arid southwestern United States. Basic field and isotopic dating methods are combined with two surface‐dating techniques, cosmogenic chlorine‐36 and rock varnish microlaminae, to produce a detailed slope development history with broad implications for geomorphic theory that includes the unresolved geomorphic debate between Walther Penck and William Morris Davis. Slopes in this study are dominated by the on‐going desert slope processes of debris flows and in‐situ grain disintegration as evidenced by active debris flow features, terminal Pleistocene ages of microlaminae, and chlorine‐36 ages progressively younger than potassium‐argon ages for slope genesis. Results also indicate that slopes retreat in a parallel fashion as postulated by Penck. Furthermore, the deposits do not exhibit significant changes in grain size, shape, or angularity from genesis to ～0.6 Ma but change markedly after that time possibly indicating a geomorphic threshold between ～0.6 and ～1 Ma, or episodic erosional events throughout the mid to late Pleistocene.     

SPECIFIC EROSIONAL AND DEPOSITIONAL PROCESSES IN A PLEISTOCENE SUBGLACIAL TUNNEL IN THE WIELKOPOLSKA REGION,POLAND

The Pleistocene Cie?le succession accumulated in a subglacial tunnel and shows three sedimentological units: (1) trough cross‐stratified sand with granules deposited in deep channels up to 5.4 m, (2) trough stratified and massive gravels deposited in a very deep channel up to 6.2 m eroded by a catastrophic hyperconcentrated flow, and (3) a massive diamicton, interpreted as a basal till of melt‐out type. We focus on angular and deformed sandy clasts that occur in the second unit. It appears that thermal erosion, short transport in a sediment‐laden current and sudden sedimentation were responsible for the oversized sandy clasts that occur in the gravel glaciofluvial deposits. The deposits are characterized by large‐scale erosional scours, massive structure, and fluid‐escape deformations. This combination of features can be used as a key tool for the interpretation of hyperconcentrated‐flow conditions beneath Pleistocene ice sheets.     

Middle-Late Pleistocene polycyclic evolution of a stable coastal area (southern Apulia, Italy)

The Otranto–Leuca coastal tract is marked by the presence of numerous sea caves placed close to present sea level. They are located generally at the back of a shore platform covered by a sequence of breccia deposits, marine sediments and speleothems. At Grotta di Masseria dell'Orte, marine cemented sands rest on a narrow shore platform at about 6.2 m above mean sea level and are covered by speleothems older than 185 ka. At Grotta del Diavolo, which is mostly filled by breccia deposits, three beach levels have been detected at about 3.0, 3.5 and 5.9 m above msl. They are either covered by or overlie speleothems that yield an U/Th age of 340, 78 ka and between 170.3 and 146.5, respectively. Geomorphological evidence and radiometric ages indicate that the area after a period of uplift has been tectonically stable since the last part of the Middle Pleistocene so that marine landforms close to the present shoreline underwent a polycyclic evolution. The sedimentary fills of sea caves formed during Middle-Late Pleistocene glacial stages, when arid or semiarid conditions promoted the removal of regolith and the development of thick breccia deposits. During Marine Isotope Stages (MIS) 9.3, 5.5 and 5.1, cave sediments were partially eroded whereas beach layers and related speleothems developed. These are, in fact, the only marine isotope stages marked by a sea level position which in this Mediterranean region was either close to, or slightly higher than, the present one.     

Late Devensian and Holocene landscape change in the uplands of the Isle of Man

The Late Glacial and Holocene geomorphology of the Manx uplands has received scant attention in previous researches. Solifluction deposits and terraces provide the earliest evidence for geomorphic activity after deglaciation. Fluvial incision into drift-choked valleys is correlated with the formation of the large mountain front alluvial fans that flank the Manx uplands. Formation of these alluvial fans is constrained to 15,000–10,500 cal. years BP by ¹⁴C dates on organic deposits beneath and above the alluvial fan gravels. Alluvial fan and river terraces along four valleys postdate this incision. Optically Stimulated Luminescence (OSL) and ¹⁴C dating provide a tentative chronology for these landforms. The higher terraces are Late Glacial fluvial surfaces that were probably occupied by rivers into the Holocene. Incision during the Late Holocene led to the abandonment of the higher surfaces, producing a suite of younger river terraces and alluvial fan surfaces. Independent dating constrains this fluvial activity to post-Bronze Age (3500–2800 cal. years BP). Increased human activity and climatic change during the Late Holocene are possible causes for this increased geomorphic activity.     

Geomorphological Holocene events as deduced from lithological records of the fluvial basins of Dauria

A conjugate analysis of the processes of morphogenesis and sedimentation within small fluvial basins of Dauria revealed a rhythmic character of change of the Holocene processes which are combined into cycles of a different hierarchical level. It is shown that each cycle begins with an extreme phase of intense fluvial drift and delivery of matter from the system. The fluvial horizons comprise the lower part of deposits of the Holocene macrocycle in the Vesnyanka pad’ (a valley without a permanent watercourse). A disastrous fluvial drift was recorded repeatedly for the period 7–8.5 cal. ka and at the transition time from the Atlantic to Subboreal time. A normal zonal phase of integration of matter within the basins is exhibited by buried humus horizons of soils which give clear evidence of 1800-year cycles. A regional development in Dauria is recorded for the soils with the age of about 8, 6.4 and 4.6 as well as 1.2–1.4 cal. ka. The upper part of the deposits of each cycle is characterized by aeolian horizons of extreme arid phases. The profiles of proluvial trails are indicative of 130-year cycles. Based on results from studying buried soils, data of pollen analysis and radiocarbon dating of the deposits, we determined an enhancement in climate aridization for the last 10 ka and, in general, a decreased of the sedimentation and morphogenesis rate.