This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Limestones, as a rule, form bold massifs or hills, and they are not covered by a great thickness of soil. Indeed, large areas are denuded of soil, forming pavements, and upon these plants grow as upon the slopes where detritus has accumulated.
1 Cf. also Section IV.
The natural type of woodland upon limestones is Ash wood, which ranges up to an altitude of 1000 ft. Associated with the Ash are the Wych Elm and Hawthorn, the former at lower elevations in damp situations, the latter higher up in dry situations, and either may replace the Ash locally.
Juniper and Yew are also found on calcareous soils. The Hazel is common, but the Alder is local. In some areas the Birch is found. Other members of tree or scrub zones are Lime, Buckthorn, Spindle Tree, Scotch Rose, Eglantine, White Beam (as on chalk), Cornel, Wayfaring Tree, Privet, Spurge Laurel. Heaths and Ling are absent. In damp spots the herbaceous types are Marsh Marigold, Meadow-sweet, Water Avens, Valerian, Butterbur, Melancholy Thistle, Jacob's Ladder, Scorpion Grass, Bur Reed, Reed, etc.
On dry soils Dog's Mercury and Moschatel are very characteristic types, and in damper places Yellow Archangel, Ransoms, Bellflower. Ground Ivy, and, where the ground is stony, Hairy St. John's Wort, Nettle, Wood Sage, Lily-of-the-Valley, Melic Grass.
Rocky knolls occur where there is little or no soil and bare rock, with cryptogams, Sandwort, Whitlow Grass, Rue-leaved Saxifrage, Biting Stonecrop, Thyme. The woodland ceases where the rocks outcrop, except on the pavements. The screes and cliffs, when damp, are colonized by the vegetation of the Ash wood itself.