Codwyd tref Porthaethwy ar greigiau 'schist' Cyn Gambriaidd sydd oddeutu 600 miliwn o flynyddoedd oed, a ffurfiwyd pan roedd rhan yma y byd yn yr Hemisffer ddeheuol! Ceir rhai llecynnau o greigiau folcanig igneaidd) caletach hefyd a fwyngloddwyd o chwareli bychain yn y dref yn y gorffennol ar gyfer adeiladu. Cafwyd chwarel yn Tyddyn To ac hefyd ger Pont Menai.

Mae uchder y tir yn amrywio o 0m ger glannau'r Fenai hyd at 152m uwch lefel y mor ger trogylch Four Crosses ar Ffordd Pentraeth.

Eithaf gwastad yw'r tir ger rhannau hynaf y dref wrth y Fenai ond mae'r rhannau newydd, yn bennaf stadau tai, wedi eu hadeiladu ar y llethrau serth sy'n wynebu'r de.

Gwelir felly bod Porthaethwy wedi datblygu ar i fyny'r bryn gan nad oedd modd iddi ehangu i'r gorllewin, yn rhannol oherwydd y llethrau serth sy'n arwain i lawr at y Fenai ac yn rhannol oherwydd y cyfyngderau a sefydlwyd gan berchnogion Plas Newydd (Marcwis Mon) sydd berchen y tir. Nid oes modd i'r dref ehangu i'r dwyrain chwaith, oherwydd llethrau serth Dyffryn Cadnant ac yn rhannol o ganlyniad i reolau adeiladu a sefydlwyd gan Stad Baron Hill (Teulu'r Bulkley) o Fiwmares, sydd berchen y tir ar yr ochr honno.

Efallai, yn y dyfodol bydd y dref yn ehangu i'r dwyrain ac i'r gorllewin tuag at Llandegfan a Llanfairpwll gwyngyll, ar hyd rhan uchaf y llethr, ond teg yw dweud bod caniatad cynllunio wedi ei wrthod ar gyfer y mannau hyn yn ddiweddar.

Menai Bridge is built on very old rocks, mainly Precambrian schists, probably more than 600 million years old and formed when this part of the world was still in the Southern Hemisphere! There are some areas of harder volcanic rock too, quarried from tiny quarries around the town in the past for building stone e.g. at Tyddyn To and also near where Menai Bridge is.

The height of the land ranges from 0m on the Straits to 152m above sea level, near the Four Crosses roundabout on Pentraeth Road. The older part of the village is fairly flat, near the Straits, but the newer parts, mostly housing estates are built on steep slopes facing south.

It can be seen therefore that the town has grown and spread up the hill because it could not spread west, partly because of the very steep slopes leading down to the Straits, and partly due to restrictions laid down by the owners of Plas Newydd who own the land (the Marquis of Anglesey). It was not possible for the town to extend eastwards either because of the steep slopes of the Cadnant Valley and partly due to the building restrictions laid down by the Baron Hill estate (The Bulkley family) of Beaumaris who own the land on that side.

Perhaps, in the future the town may spread West and East towards Llanfairpwllgwyngyll and Llandegfan respectively, along the top of the slope, but recently, it is fair to say that planning permission has been declined for many developments in these areas.