It is possible that the spread of these beetles was coincidental, although the hypothesis has also been suggested that farmers intentionally spread the beetles so that they destroyed the elm forests, providing more deforested land for farming. Meanwhile, during the period from around 3500 to 3300 BCE, many of these deforested areas began to see reforestation and mass tree regrowth, indicating that human activity had retreated from these areas. Around the period between 3500 and 3300 BCE, agricultural communities had begun centring themselves upon the most productive areas, where the soils were more fertile, namely around the Boyne, Orkney, eastern Scotland, Anglesey, the upper Thames, Wessex, Essex, Yorkshire and the river valleys of The Wash. These areas saw an intensification of agricultural production, and larger settlements. The Neolithic houses of the British Isles were typically rectangular, being made out of wood, and as such none survive to this day. Nonetheless, foundations of such buildings have been found in the archaeological record, although they are rare, and have usually only been uncovered when they were in the vicinity of the more substantial Neolithic stone monuments. The Early and the Middle Neolithic also saw the construction of large megalithic tombs across the British Isles. Because they housed the bodies of the dead, these tombs have typically been considered by archaeologists to be a possible indication of ancestor veneration by those who constructed them. Such Neolithic tombs are common across much of western Europe, from Iberia to Scandinavia, and they were therefore likely brought to the British Isles along with, or roughly concurrent to, the introduction of farming. A widely held theory amongst archaeologists is that these megalithic tombs were intentionally made to resemble the long timber houses which had been constructed by Neolithic farming peoples in the Danube basin from circa 4800 BCE. As the historian Ronald Hutton related, "There is no doubt that these great tombs, far more impressive than would be required of mere repositories for bones, were the centres of ritual activity in the early Neolithic: they were shrines as well as mausoleums. For some reason, the success of farming and the veneration of ancestral and more recent bones had become bound up together in the minds of the people." Although there are disputed radiocarbon dates indicating that the chambered tomb at Carrowmore in Ireland dates to circa 5000 BCE, the majority of such monuments in the British Isles appear to have been built between 4000 and 3200 BCE, a time period that archaeologist Mike Parker Pearson notes means that tomb building was "a relatively short-lived fashion in archaeological terms."