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1. Upper Part of Main Valley
1. A. Stable Shingle
The areas of shingle in the upper part of the main valley that were undisturbed by the heavy floods in October were regarded as being stable at the time of the investigation. These areas occurred both as shingle beaches (banks) and as beds or islands in midstream. They were usually covered with a mixed herbaceous vegetation, although on some banks grasses were predominant. In the mixed herbaceous community Digitalis purpurea was the most prominent plant, and associated with it were the large dock Rumex obtusifolius and two species of Chrysanthemum—C. leucanthemum and C. parthenium. The most abundant grasses were Dactylis glomerata and Anthoxanthum odoratum, and the rushes Juncus bufonius and J. lampocarpus were locally abundant.
Considerable amounts of silt often accumulate between the stones and the density of the ground cover varies with the amount of silt. Where there was only a small amount of soil and the taller vegetation was relatively open, the creeping plants Geranium molle, G. Robertianum, Acaena sanguisorbae and Epilobium nummularifolium formed a close ground mat; but in areas where silt had accumulated there was a higher percentage of grasses forming a closed turf which included Dactylis glomerata, Anthoxanthum odoratum, Holcus lanatus and Poa annua. In wet depressions and swampy areas Agrostis stolonifera and
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Fig. 9.—Shingle disturbed and reported on 26th October. 1951.
Fig. 10—The same area 14 months later December 1952
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in flower young plants of
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and the grasses
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and
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9—Shingle disturbed and reported on 26th October. 1951.
Fig. 10—The same area 14 months later December 1952
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in flower young plants of
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and the grasses
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and
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Fig. 11.—Colonization of shingle by
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Fig. 12.—Colonization of a salt deposit
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with a relatively undisturbed area of older shingle marked by grasses
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and
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in the background.
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Fig. 1.—Summary of the changes in a shingle bank in the middle reaches of the stream, since its deposition in 1947 until 1952.
Fig. 2.—Distribution of plant communities on this unstable shingle bank before the heavy flood in October, 1951.
Fig. 3.—Changes in the shingle area and the vegetation after the flood.
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Cotula coronopifolia were the most common plants with Juncus effusus and Rumex obtusifolius. In small pools and along the stream margin the characteristic plants were Glyceria fluitans, Mimulus guttatus and Callitriche verna.
In contrast, in the more stable lower parts of the stream, many of the small shingle beaches had a closed turf cover in which Agrostis alba, A. stolonifera and Anthoxanthum odoratum were prominent. After heavy floods some of these small beaches were still covered with water when most areas were dry, but they were unaltered in extent or vegetation when the water receded. These areas were regarded as examples of what is, in all probability, the final stage in stabilization of the shingle deposits.