This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The sand and shingle cast up by the agency of the sea is of purely modern origin. Marine deposits in fact belong to the latest geological period, the recent or Quaternary epoch.
It is also known that England was connected with the Continent in Preglacial times, and during some part of the Glacial period, hence the present maritime flora is of comparatively modern type. Apart from the records of a few present maritime plants in Glacial and later beds the succession is broken, for apart from Naias, which occurs in the Preglacial Cromer Forest Bed, there are no earlier records of marine types that are connected with our present maritime flowering plants.
Amongst those described, two are found in the Oak Zone in Scandinavia, Sea Rocket and Sea Buckthorn; three in Interglacial beds in this country, Sea Campion (found to-day, inland, on the hills), Sea Purslane, Sea Milkwort; and the Sea Club Rush is found in Neolithic beds. The raised beaches and shingle beaches found inland are of modern origin also. Isolation of Maritime Types on Hills. - The occurrence of certain typical maritime plants upon inland mountains, and their absence from intervening lowlands, constitute one of the most curious facts of plant distribution. Discontinuous distribution, as shown by Dr. A. R. Wallace, is an indication of the antiquity of the plant types so dispersed. Hence it may be considered that the occurrence of such plants as Scurvy Grass, Thrift, Sea Plantain, Sea Campion, etc., inland on lofty mountains is connected with the existence in earlier times of sea margins, of which there are indications (not where these plants occur) elsewhere at a distance from the sea.
There are other reasons for the occurrence of such types inland away from the sea. It is quite possible that the seeds of such plants may be conveyed thither by animal agency, especially birds. Wild fowl, ducks, geese, swans, plovers, and many other birds frequent the coast on migration, and also inland lochs and upland moors and bogs. Some maritime birds, as the Dotterel, Redshank, Blackheaded Gull, and others, leave the sea-coast during the breeding season to seek such habitats as the inland types of maritime plants frequent.
Whatever be the cause of the isolation of these maritime plants upon hills inland, their occurrence in such spots is of the greatest interest, and any observations that may be made upon these points will assuredly be prolific in results.
In the case of the montane types of maritime plants described in the last section, there were no conditions inland resembling those on the sea-coast to account for the occurrence of Thrift, etc, in alpine situations. In the case of certain inland salt-marshes, however, the conditions are similar to those on the sea-coast; that is to say, they are suited to halophytes, plants that require salt or brackish water. In the neighbourhood of Droitwich, famous for its brine springs, such plants are to be found, also in the Severn valley from the Salwarpe valley to Droitwich, and in the Tewkesbury and Evesham district.
At Longdon and Welland marshes Cenanthe pimpinelloides, Oe. lachenalii, Ce. silaifolia (all maritime species of Dropwort), Golden Dock, Sea Club Rush, and Alexanders occur. If the land were submerged 100 ft., the tide which comes far up the Severn would reach this point. Woad was found on cliffs also in the Severn valley. Parsley, Fennel, Soapwort, and Carum segetum also grow at Dodder-hill in the same region. Dittander, a salt-marsh plant, occurs at Salwarpe, and Senecio squalidus at Droitwich.
Between Droitwich and Hawford Lock grow Celery, Sea Milkwort, Sea Spurrey, Sea Orache, Sea Arrow Grass, Sea Club Rush, Juncus compressus, and Brookweed. In the valley of the Trent, near Stratford, 250 ft. above the sea, also grow Sea Spurrey, Celery, Sea Aster, Sea Milkwort, Sea Arrow Grass, Sea Club Rush. Sea Stork's Bill is also found in various parts of Worcestershire. Wherever these plants occur there are salt springs, hence their occurrence inland. They may have been introduced by birds that frequent maritime salt marshes.