Excavation has revealed that some of the long barrows in the area of modern Spain, Portugal, and western France were erected in the mid-fifth millennium BCE, making these older than those long barrows further north.[24] Although the general area in which the oldest long barrows were built is therefore known, archaeologists do not know exactly where the tradition started nor which long barrows are the very first ones to have been built.[24] It therefore appears that the architectural tradition developed in this southern area of Western Europe before spreading north, along the Atlantic coast.[24] The tradition had reached Britain by the first half of the fourth millennium BCE, either soon after farming or in some cases perhaps just before it.[24] It later spread further north on mainland Europe, for instance arriving in the Netherlands in the second half of the fourth millennium BCE.[24]