Here are the relevant biblical texts, before we begin our survey of the history of the excavations on Mt. Ebal:
Exodus 20:24-26 (RSV) An altar of earth you shall make for me and sacrifice on it your burnt offerings and your peace offerings, your sheep and your oxen; in every place where I cause my name to be remembered I will come to you and bless you.  And if you make me an altar of stone, you shall not build it of hewn stones; for if you wield your tool upon it you profane it.  And you shall not go up by steps to my altar, that your nakedness be not exposed on it.'
Deuteronomy 27:1-13 Now Moses and the elders of Israel commanded the people, saying, "Keep all the commandment which I command you this day.  And on the day you pass over the Jordan to the land which the LORD your God gives you, you shall set up large stones, and plaster them with plaster;  and you shall write upon them all the words of this law, when you pass over to enter the land which the LORD your God gives you, a land flowing with milk and honey, as the LORD, the God of your fathers, has promised you.  And when you have passed over the Jordan, you shall set up these stones, concerning which I command you this day, on Mount Ebal, and you shall plaster them with plaster.  And there you shall build an altar to the LORD your God, an altar of stones; you shall lift up no iron tool upon them.  You shall build an altar to the LORD your God of unhewn stones; and you shall offer burnt offerings on it to the LORD your God;  and you shall sacrifice peace offerings, and shall eat there; and you shall rejoice before the LORD your God.  And you shall write upon the stones all the words of this law very plainly."  And Moses and the Levitical priests said to all Israel, "Keep silence and hear, O Israel: this day you have become the people of the LORD your God.  You shall therefore obey the voice of the LORD your God, keeping his commandments and his statutes, which I command you this day."  And Moses charged the people the same day, saying,  "When you have passed over the Jordan, these shall stand upon Mount Ger'izim to bless the people: Simeon, Levi, Judah, Is'sachar, Joseph, and Benjamin.  And these shall stand upon Mount Ebal for the curse: Reuben, Gad, Asher, Zeb'ulun, Dan, and Naph'tali. (cf. 11:26-29)
Joshua 8:30-35 Then Joshua built an altar in Mount Ebal to the LORD, the God of Israel,  as Moses the servant of the LORD had commanded the people of Israel, as it is written in the book of the law of Moses, "an altar of unhewn stones, upon which no man has lifted an iron tool"; and they offered on it burnt offerings to the LORD, and sacrificed peace offerings.  And there, in the presence of the people of Israel, he wrote upon the stones a copy of the law of Moses, which he had written.  And all Israel, sojourner as well as homeborn, with their elders and officers and their judges, stood on opposite sides of the ark before the Levitical priests who carried the ark of the covenant of the LORD, half of them in front of Mount Ger'izim and half of them in front of Mount Ebal, as Moses the servant of the LORD had commanded at the first, that they should bless the people of Israel.  And afterward he read all the words of the law, the blessing and the curse, according to all that is written in the book of the law.  There was not a word of all that Moses commanded which Joshua did not read before all the assembly of Israel, and the women, and the little ones, and the sojourners who lived among them.
The central figure in our story is Dr. Adam Zertal, Professor, Dept. of Archaeology at the University of Haifa (and its chairman from 1996-1999). He received his Ph.D. from Tel Aviv University in 1988. His dissertation was entitled, “The Israelite Settlement in the Hill Country of Manasseh”.1
He is also the author of the entries on Mt. Ebal in The New Encyclopedia of Archaeological Excavations in the Holy Land (edited by Ephraim Stern, Jerusalem: 1993; see pp. 375-377), and The Oxford Encyclopedia of Archaeology in the Near East (edited by Eric M. Meyers, Oxford Univ. Press, 1996; see pp. 179-180).
Additionally, Zertal has written five books in Hebrew about the hill country of Manasseh, from 1988 to 1999. Zertal described his earlier positions in a 2010 interview:
I spent a year at Hadassah Hospital in Jerusalem [having been severely injured in the 1973 Yom Kippur War], and I became interested in archeology. Although I had argued that the Bible was full of myths, I decided after my recovery to travel the land by foot to look for archeological evidence. . . . I am a man of science and have to investigate whether what is described in the Bible suits the geography. Nobody thought there was an altar on Mount Ebal, but the evidence was found. It is not a legend. When you do archeological research as you should, you see a lot [of the biblical stories] is reality. 2
Zertal certainly knows the geographical area that he specializes in very well. The same article notes that:
. . . for 33 years [he] has led weekly walks with university colleagues and volunteers over 'every square meter' of Samaria and the Jordan Rift to search for archeological evidence from biblical times.
When he made his discovery in April 1980, he was not inclined to support biblical texts at all. He stated later:
At that time I never dreamt that we were dealing with the altar, because I was taught in Tel Aviv University - the center of anti-Biblical tendencies, where I learned that Biblical theories are untrue, and that Biblical accounts were written later, and the like. I didn't even know of the story of the Joshua's altar. But we surveyed every meter of the site, and in the course of nine years of excavation, we discovered a very old structure with no parallels to anything we had seen before.3
Dr. Zertal published his initial findings and conclusions along these lines in his article, “Has Joshua’s Altar been Found on Mount Ebal?”, Biblical Archaeology Review XI (1985), pp. 26-44. I shall both cite and summarize this striking piece (it can be read on Steve Rudd's web page: see footnote 3):
On a cool spring afternoon in April-April 6, 1980, to be exact-when we had nearly completed our survey of the mountain, we came upon a large heap of stones that was not very different from the thousands of stone heaps we had already found, collected by farmers as they cleared their fields for planting. True, the stone heap was somewhat larger than the typical one, but what really distinguished it was the great quantity of pottery sherds lying around it.
We were immediately able to date these sherds to the early part of the period archaeologists call Iron Age 1 (1220-1000 B.C.), the period during which the Israelites entered Canaan and settled there. Iron Age 1 also includes the period of the Judges.
. . . It took us two years to raise funds to excavate the heap of stones, and to organize our expedition. But I must confess we did not rush, for we never dreamed that the site would prove to be the earliest and most complete Israelite cultic center ever discovered and the prototype of all later ones. It took us another two years and three seasons of digging to find out what we were really excavating.
What he found was a nearly square structure, almost nine feet high, and about 25 by 30 feet in width and length. Zertal's first theory was that it was perhaps a “watchtower” or a “farmhouse.” But it was not like any other farmhouse in the area that he was familiar with. It had no entrance. He also ruled out the watchtower theory, since he saw no reason for one to be there. No Iron Age settlement was nearby. Evidence then started surfacing as to its function as an altar:
. . . the bones, which were found in such large quantities in the filling, were sent for analysis to the zoology department of the Hebrew University in Jerusalem. The bones proved to be from young male bulls, sheep, goats and fallow deer. . . . The first chapter of Leviticus describes the animals that may be offered as sacrifices. A burnt offering must be a male without blemish (Leviticus 1:3). It may be a bull (Leviticus 1:5) or a sheep or a goat (Leviticus 1:10). The close match of the bones we found in the fill with this description in Leviticus 1 was a strong hint as to the nature of the structure we were excavating.
. . . 942 bones were examined, representing 50-100 specimens. These were attributed to four kinds of animals: goats, sheep, cattle, and fallow deer. The latter is a light-spotted animal which inhabited the woodlands of our country in antiquity. Examination of the sex and age of the animals revealed that all those that could be diagnosed were young males, approximately one year old. This correlates remarkably with the laws of sacrifice in the book of Leviticus:
And the Lord called unto Moses, and spoke unto him out of the tent of meeting, saying: Speak unto the children of Israel, and say unto them: When any man of you bringeth an offering unto the Lord, ye shall bring your offering of the cattle, even of the herd or of the flock. If his offering be a burnt-offering of the herd, he shall offer it a male without blemish" (Leviticus 1:1-3).
A great part of the bones, as we mentioned, had been burned over a fire and were cut near the joints. Being scorched in this way attests that the flesh was not intended for eating but was burned over an open fire (i.e. not in an oven). Thus the high correlation with the biblical laws of sacrifice, together with the great architectural resemblance to Israelite altars, confirmed the view that we were dealing with a cultic site and altar from the beginning of the Israelite settlement.
The Hebrews were allowed to eat deer:
Deuteronomy 14:4-5 These are the animals you may eat: the ox, the sheep, the goat,  the hart, the gazelle, the roebuck, the wild goat, the ibex, the antelope, and the mountain-sheep.
It was also discovered that underneath the center of the structure was an older circular stone formation of about 6.5 feet in diameter. Some have speculated that this was Joshua's altar, over which a later altar was built.
Moreover, the structure consisted of several courts. In these were found bones of animals that had not been burned, and where the animals were eaten. This lined up with Deuteronomy 27:7 (see above). The ramp up to the top also corresponds to Exodus 20:26 (above). Zertal describes it:
A ramp of unhewn stones, 4 feet wide by 23 feet long, rises to the top of the platform from the southwest. The gentle incline, easily climbed . . . the ramp on our Mt Ebal altar indicates a strict adherence to the law in Exodus 20:26, which requires a ramp rather than steps: . . .
Zertal then goes into a detailed description of altars as described in the Bible, and comparisons to those of other non-Hebrew ancient near Eastern altars. Everything fits nicely into the theory that the structure on Mt. Ebal is, in fact, an early Hebrew altar. He makes note of another factor suggesting an early Israelite date:
Every other ancient altar that has been discovered thus far, however, was connected with a temple, or as at Beer-Sheva, was in a city where we may suppose a temple existed in connection with the altar (2 Kings 23:8). Our altar alone seems to have been an independent altar in the country side, not associated with a temple or a settlement. This is probably because the Mt. Ebal altar and its associated cult site were built at a very early period in the development of Israelite cult and religion; at that time, there was no temple. Moreover, the Mt. Ebal cult center lasted for only a relatively short time. It is unlikely that a temple could develop in such a short time. Even at Shiloh, which was the site of the successor to the Mt. Ebal cult center, no temple was built.
With respect to the Mt. Ebal altar, . . . all the scientific evidence fits very well with the Biblical description. The three main factors that correlate precisely are the period, the nature of the site, and the location. True, no inscriptions have been found as yet. But apart from that one point, it may be said with all scientific restraint that there must be a connection between the strong, important and authentic Biblical tradition that identifies Mt. Ebal as a central Israelite cultic center and the gathering place of the Israelite tribes, on the one hand, and the site unearthed by us, on the other. . . . We have on Mt. Ebal not only the complete prototype of an Israelite altar, but moreover, a site that might prove to be directly related to the Biblical traditions concerning Joshua's building of an altar on Mt. Ebal.
Other evidences of corresponding dates were also found: an “Egyptian-style scarab” which is determined through five other known parallels to date from the 13th-12th century B. C. Other distinctive forms of pottery found, belong to the same period. Most remarkably, this scarab and others found at the location “date to the time of the great Egyptian pharaoh, Ramses II, who is considered the pharaoh of the exodus from Egypt.”
In November 2004, Dr. Zertal made additional comments and conclusions about the Mt. Ebal excavations, updating his earlier ones. He notes the consensus that has been established in archaeology and continuing skepticism in considerable sectors of that community:
No scholar challenges the fact that this is an extremely important and authentic tradition dealing with a central event in the life of the people. All agree that this event took place on Mt. 'Ebal. As to the date of the event and the date it was recorded, however, views vary. . . . The central altar was erected on Mt. 'Ebal, and there Israel became "a people unto the Lord thy God" (Deuteronomy 27:9); . . . Reputable scholars have suggested that the entire story of the conquest is nothing more than a later, etiological tradition which sets out to account for various manifestations in the light of mythological traditions and folklore. Recent extensive archaeological surveys of the central hill country, however, reveal clearly the process of Israelite settlement as a major settlement movement of the era (1250-1100 b.c.e.). Hundreds of newly-founded, small settlements were established within a short period throughout the hilly allotments of the tribes of Manasseh, Ephraim and Benjamin. The settlers used a characteristic type of pottery and their houses were generally built on a three- or four-room plan.
. . . The cultic site on Mt. Ebal satisfies the three criteria necessary to identify a biblical site: chronological (beginning of the Israelite settlement), geographical, and the nature of the site (a cultic center with a burnt-offering altar). In view of this analysis, the identity of the biblical story and this site as the first inter-tribal center of the Israelite tribes can hardly be doubted. This is the first time a complete Israelite cultic center, including an altar for burnt offerings, is available for study. . . . The altar on Mt. 'Ebal is not only the most ancient and complete altar, but also the prototype of the Israelite burnt offering altar of the First and Second Temple periods. The Mesopotamian architectural influence on the structure of the altar is also very interesting, both in its stepped construction and in the orientation of its corners to the north, south, east, and west.
. . . The varieties of animal bones discovered are evidence that the laws of sacrifice were followed from the very beginnings of the Israelite religion. Despite the presence of wild boars in the region, not a single bone of this animal, not fit for sacrifice, was found on Mt. 'Ebal.4
Smithsonian Magazine took note of Dr. Zertal's claims in May 2006.5 He is cited as saying, ““The altar was supposed to be nonexistent, a legend,” and the writer comments on the rampant biblical skepticism within archaeology (which is detailed at length):
In this search, the Old Testament has quite literally been his guide. This approach was once common for archaeologists in Israel, but in recent years it has come to define an extreme position in a debate over whether the Bible should be read as historical fact or metaphorical fiction.
Those in Zertal’s camp say that all, or nearly all, the events in the early books of the Old Testament not only actually happened but are supported by material evidence on the ground. On the other side are the so-called biblical minimalists, who argue that the Old Testament is literary rather than historical—the work of ideologues who wrote it between the fifth and second centuries b.c.—and that Moses, Joshua, David and Solomon never even existed. A third group accepts the Bible as folk memory transmuted into myth—a mixture of fact and fiction. They argue over the balance between the two.
. . . For the literalists, the stones at Mount Ebal are crucial. “If this corroborates exactly what is written in that very old part of the Bible,” says Zertal, “it means that probably other parts are historically correct. The impact is tremendous.”
Bible scholar and commentator Pekka Pitkänen (whose doctoral work was devoted to very similar areas of study) defends in several respects the findings of Dr. Zertal:6
[W]hen scholars object to the possibility of interpreting the site as Joshua's altar based on a reading of the book of Joshua, they are not proceeding on an archaeological basis, but replacing one literary reading of the biblical text with another . . .
. . . if we think that the exodus/early settlement happened in the thirteenth century, it should rather be this altar [the round one lower in the strata] that should be associated with Joshua, if anything. Zertal himself thinks that the older altar was part of a foundation ceremony before the building of the actual altar (A. Zertal, personal communication, December 1999).
. . . What about the plastered stones? . . . one has to stress the fact that finding plaster at the site is extraordinary. [see Dt 27:2, 4 above]
. . . the uniqueness of the main structure with its surrounding wall complex and its possible connections with Joshua make the question of the nature of the site at Mount Ebal nothing less than intriguing. Also the fact that no structure has been found at Mount Ebal from Iron Age II rather speaks of the antiquity of the Joshua tradition, as there is no evidence of a cultic centre at Mount Ebal during the time of the monarchy from which to draw the tradition.7
Richard S. Hess, Professor of Old Testament and Semitic Languages at Denver Seminary (Ph.D. from Hebrew Union College), and author of over 100 scholarly articles, claims in his commentary on Joshua:
[A]fter reading the excavator's report and conducting a visual tour of the site, it certainly looks like an early Israelite altar such as is described in this text in Joshua. Despite strong opposition by others, there remains no better explanation than that this represents an anomalous Early Iron Age cultic site that has no clear cultural antecedents anywhere in the region.8
Dr. Hess's extraordinary academic achievements in Old Testament study9 give his opinion considerable weight. The authors of A Biblical History of Israel10 also essentially agree with Zertal:
[W]hen the full body of evidence is considered, the conclusion that the site seems more like a cult installation than like anything described by competing theories is hard to deny. . . . on balance, Zertal's cultic theory may well prevail.
Kenneth A. Kitchen11, the eminent Egyptologist and archaeologist, formerly of the University of Liverpool; author of over 250 books and articles on such topics since the 1950s, believes that the farmhouse theory is ruled out and that the watchtower hypothesis is “feasible” but nevertheless “not beyond objection” and “open to some doubt.”
He doesn't take a final position on Zertal's opinion, and writes, “There is no final proof or disproof for either a watchtower or an altar complex (of Joshua or otherwise).” But he strongly critiques the closed-mindedness of Zertal's vocal critics:
It is noteworthy that the fiercest opposition to the specter of Joshua's altar has come from minds not open to such revolutionary possibilities. Thus, all that Kempinski could finally offer against the concept was the old views about the theoretical late (Deuteronomic) date for the books of Deuteronomy and Joshua in the seventh century which are not fact, merely dogma. . . . To Rainey's charge that only the gullible would believe Zertal's claim, one may observe that such people as Coogan and Mazar (who both grant a cultic possibility) could hardly be thus dismissed. Colorful language is not the answer either. In short, Zertal's views is feasible, but absolute certainty eludes us.12
We see, then, that prior hostile bias and academic egos are in full display within biblical and Palestinian archaeology (as we would fully expect). In my “non-scholarly” opinion, for whatever it's worth, I think a good deal of confirming evidence is in play, consistent with the related biblical texts to an extraordinary degree. I agree that it's not absolutely “proven” to be Joshua's altar, but few things admit of absolute proof, so that doesn't concern me, and I am most impressed by the cumulative archaeological evidence.
1 Gilgal Education Center: Professor Adam Zertal; University of Haifa (http://www.gilgalvisitorcenter.org/wp/professor-adam-zertal-2/)
2 “Christian in Israel: Long time archaeological riddle solved,” by Judy Siegel-Itzkovich, The Jerusalem Post, 2 July 2010.
3 “Joshua's Altar on Mt. Ebal, Israel”; extensive web page by archaeology buff Steve Rudd (http://www.bible.ca/archeology/bible-archeology-altar-of-joshua.htm)
5 “Shifting Ground in the Holy Land Archaeology is casting new light on the Old Testament,” by Jennifer Wallace (http://www.smithsonianmag.com/history/shifting-ground-in-the-holy-land-114897288/?page=1)
6 Joshua [Apollos Old Testament Commentary], (Downers Grove, Illinois: InterVarsity Press, 2010): “Excursus 7: The Archaeology of Mt. Ebal,” pp. 192-214. The author drew heavily from his earlier work, Central Sanctuary and Centralization of Worship in Ancient Israel: From The Settlement To The Building Of Solomons Temple (Piscataway, New Jersey : Gorgias Press, 2004): his doctoral dissertation for the University of Gloucestershire.
7 Ibid., pp. 200, 202-204.
8 Zondervan Illustrated Bible Backgrounds Commentary: Joshua, Judges, Ruth, 1 & 2 Samuel, John H. Walton, general editor (Grand Rapids, Michigan: Zondervan, 2009), p. 39.
9 See his Curriculum Vitae: (http://www.denverseminary.edu/about/faculty/member/13474/); also his comments in his book, Israelite Religions: An Archaeological and Biblical Survey (Grand Rapids, Michigan: Baker Academic, 2007), pp. 216-219 [including two great close-up photographs]; available to read online at Google Books.
10 Iain Provan, V. Philips Long, Tremper Longman III, (Lousiville: Westminster John Know Press, 2003), p. 186.
12 On the Reliability of the Old Testament (Grand Rapids, Michigan: Wm. B. Eerdmans Publishing Co., 2003), pp. 233-234.