The Horton Plains are the headwaters of three major Sri Lankan rivers, the Mahaweli, Kelani, and Walawe.

[3] The rocks in the park belong to the Archean age and to the high series of the Precambrian era and are made up of Khondalites, Charnockites and granitic gneisses.

[4][5] The soil type is of the red-yellow podsolic group and the surface layer is covered with decayed organic matter.

[6] Many pools and waterfalls can be seen in the park, and Horton Plains is considered the most important drainage basin in Sri Lanka.

The local population who resided in the lowlands ascended the mountains to mine gems, extract iron ore, construct an irrigational canal and fell trees for timber.

A 6-metre (20 ft) pollen core extracted from a mire revealed that in the late quaternary period the area had a semi-arid climate and a species-restricted plant community.

[8] Since Sri Lanka has a long non-written history, there is a significant and logical folk story, which also goes with the epic 'Ramayana' with some deviations.

It is believed that Thotupala mountain in Horton plain to be the place where King Rawana landed his aircraft, 'Dandumonaraya'.

There had been some soil tests done by local universities, and it revealed that the upper layer contains a high amount of Calcium Carbonate and Potash.

Horton Plains was designated as a wildlife sanctuary on 5 December 1969,[4] and because of its biodiversity value, was elevated to a national park on 18 March 1988.

[4] In July 2010, the Central Highlands of Sri Lanka which incorporates Horton Plains National Park, Peak Wilderness Sanctuary and Knuckles Mountain Range was inscribed on the World Heritage List.

The forest canopy reaches the height of 20 m (66 ft) and features Calophyllum walkeri, forming communities with varieties of Myrtaceae species such as Syzygium rotundifolium, and S. sclerophyllum, and Lauraceae members including Litsea, Cinnamomum, and Actinodaphne speciosa.

Species such as Gordonia and Rhododendron arboreum have spread to Sri Lanka, along the Western Ghats of South India from the Himalayas and are now common.

Waterlogged swamps or slow-moving streams are found in low-lying areas, and macrophytes such as Aponogeton jacobsenii, sedge species Isolopis fluitans and Utricularia spp.

The bamboo Chimonobambusa densifolia thrive along the banks of the streams, and near the swampy areas grass species such as Juncus prismatocarpus, Garnotia mutica, Eriocaulon spp.

Other notable plants include shrubs such as Rhodomyrtus tomentosa, Gaultheria fragrantissima, herbs, Exacum trinervium, E. walkeri, Drosera indica, and tree ferns Cyathea spp.

[2] Anzia, a foliose lichen genus belonging to the family Parmeliaceae, which had not been recorded in Sri Lanka before, was discovered here in 2007.

[6] Other mammal species found in the park include Kelaart's long-clawed shrews, toque macaques, purple-faced langurs, rusty-spotted cat, Sri Lankan leopards, wild boars, stripe-necked mongooses, Sri Lankan spotted chevrotains, Indian muntjacs, and grizzled giant squirrels.

[12] In 2016, rusty-spotted cats (Prionailurus rubiginosus) were recorded in Horton Plains National Park for the first time, at altitudes of 2,084–2,162 m (6,837–7,093 ft).

[13] Along with Ohiya, Pattipola and Ambewela, Horton Plains forms one of the Important Bird Areas (IBAs) in Sri Lanka.

[14] Together with the adjacent Peak Wilderness Sanctuary, Horton Plains contains 21 bird species which occur only on Sri Lanka.

Crested serpent eagle, mountain hawk-eagle, black-winged kite, and peregrine falcon are among the birds of prey found in Horton Plains.

They are Calotes nigrilabris, Rhino-horned lizard, Cophotis ceylanica, Lankascincus taprobanensis and common rough-sided snake.

[2] Gem mining, timber logging, the collection of plants for ornamental and medicinal purposes, encroachment, poaching and vehicle traffic are the other threats.