Twyni tywod yw’r cynefin yn bennaf, ac mae’n lloches bwysig i nifer o blanhigion a phryfed sydd dan fygythiad. Mae’r twyni’n cynnwys llecyn o dwyni tywod tir isel a llwyfandir uchel o dwyni tywod sy’n gorwedd ar ben sgarp o galchfaen. O fewn glaswelltir y twyni gellir gweld môr o flodau drwy’r rhan fwyaf o’r flwyddyn. Er mwyn helpu i reoli llecynnau agored o laswelltir y twyni, a rheoli lledaeniad
y prysgwydd a’r coetir, mae’r arfer o ddefnyddio gwartheg i bori’r tir wedi’i chyflwyno ar y warchodfa. Ceir dau lecyn mawr sydd wedi’u ffensio, ac o fewn y rhain bydd gwartheg yn pori ar adegau arbennig o’r flwyddyn. Dan y sgarp calchfaen mae ffynnon garsitig (calchog) o’r enw ‘Burrow's Well’ i’w chael mewn pant sydd wedyn yn bwydo nant fechan. Mae’r nodwedd hon yn unigryw ym Mhrydain. Yn y gaeaf, gall y dŵr sy’n llifo o’r ffynnon arwain at orlifo’r pantiau twyni cyfagos, gan gynnig cynefin ychwanegol i adar sy’n gaeafu yno. Ym Merthyr Mawr hefyd ceir un o’r poblogaethau mwyaf toreithiog o greaduriaid di-asgwrn-cefn ym Mhrydain. Mae’r cynhesrwydd sy’n deillio o’r oriau heulwen cymharol niferus, y blodau gwyllt di-rif sy’n cynhyrchu digonedd o neithdar, a’r priddoedd tywodlyd sy’n hawdd ei dyllu i adeiladu gwalau, yn gynefin delfrydol i amryw byd o bryfed. Hefyd, ceir cysylltiad cryf rhwng y warchodfa a hanes pobl. Yn wir, mae rhan fawr o’r warchodfa yn Heneb Gofrestredig. Mae’r darganfyddiadau – sy’n cynnwys fflintiau Mesolithig, bwyeill cerrig, biceri, crochenwaith Neolithig, aelwydydd o’r Oes Haearn, siambrau claddu o’r Oes Efydd a darnau a***n Rhufeinig – i’w cael ar wasgar ar draws y safle, yn hytrach nag mewn un neu fwy o safleoedd ar wahân. Merthyr Mawr Warren National Nature Reserve is approximately 340 ha of mainly sand dune habitat which provides an important refuge for many threatened dune plant and insect species. The dunes are composed of an area of lowland sand dunes and a high plateau of sand dunes that lie on top of an escarpment of limestone. Within these dune grasslands a colourful profusion of flowers can be found throughout most of the year. To help manage the open areas of dune grasslands, and control the spread of scrub and woodland, cattle grazing has been introduced to the reserve. There are two large fenced enclosures in which cattle graze at certain times of the year. Below the limestone escarpment a karstic (lime rich) spring known as Burrows Well emerges in a hollow which then feeds a small stream. This feature is unique in Britain. In winter, the outflow of water from the spring can flood the adjacent dune hollows and provide an additional habitat for overwintering birds. Merthyr Mawr also has one of the richest invertebrate populations in Britain. The warmth from the relative high number of sunshine hours, the abundance of nectar providing wild flowers, and the sandy soils in which it is easy to excavate burrows provide an ideal habitat for a wide array of insect species. There is also a large amount of human history associated with the reserve, a large part of which is also a Scheduled Ancient Monument. Finds, which have included Mesolithic flints, stone axes, beakers, Neolithic pottery, Iron Age hearths, Bronze Age burial chambers and Roman coins, are scattered over a broad area, rather than concentrated in one or more discrete sites.
