Mr. Chief Justice Burger
delivered the opinion of the Court.
The general mining law of 1872, 30 U. S. C. § 22 et seq., provides that citizens may enter and explore the public domain, and search for minerals; if they discover “valuable mineral deposits,” they may obtain title to the land on which such deposits are located.1 In 1920 Congress altered this [659]*659program with the enactment of the Mineral Leasing Act. 41 Stat. 437, as amended, 30 U. S. C. § 181 et seg. The Act withdrew oil shale and several other minerals from the general mining law and provided that thereafter these minerals would be subject to disposition only through leases. A savings clause, however, preserved “valid claims existent at date of the passage of this Act and thereafter maintained in compliance with the laws under which initiated, which claims may be perfected under such laws, including discovery.” 2
The question presented is whether oil shale deposits located prior to the 1920 Act are “valuable mineral deposits” patentable under the savings clause of the Act.
I
The action involves two groups of oil shale claims located by claimants on public lands in Garfield County, Colo., prior to the enactment of the Mineral Leasing Act.3 The first group of claims, designated Mountain Boys Nos. 6 and 7, was located in 1918. In 1920, a business trust purchased the claims for $25,000, and in 1924 an application for patent was filed with [660]*660the Department of the Interior. Some 20 years later, after extended investigative and adjudicatory proceedings, the patent was rejected “without prejudice” on the ground that it was not then vigorously pursued. In 1958, Frank W. Winegar acquired the claims and filed a new patent application. In 1964, Winegar conveyed his interests in the claims to respondent Shell Oil Company.
The second group of claims, known as Harold Shoup Nos. 1-4, was located in 1917. In 1923, the claims were acquired by Karl C. Schuyler who in 1933 bequeathed them to his surviving spouse. In 1960, Mrs. Schuyler incorporated respondent D. A. Shale, Inc., and transferred title to the claims to the corporation. Three months later, the corporation filed patent applications.
In 1964, the Department issued administrative complaints alleging that the Mountain Boys claims and the Shoup claims were invalid. The complaints alleged, inter alia, that oil shale was not a “valuable mineral” prior to the enactment of the 1920 Mineral Leasing Act.
The complaints were consolidated and tried to a hearing examiner who in 1970 ruled the claims valid. The hearing examiner observed that under established case law the test for determining a “valuable mineral deposit” was whether the deposit was one justifying present expenditures with a reasonable prospect of developing a profitable mine. See United States v. Coleman, 390 U. S. 599 (1968); Castle v. Womble, 19 L. D. 455 (1894) ,4 He then reviewed the history [661]*661of oil shale operations in this country and found that every attempted operation had failed to show profitable production. On the basis of this finding and other evidence showing commercial infeasibility, the hearing examiner reasoned that “[i]f this were a case of first impression,” oil shale would fail the “valuable mineral deposit” test. However, he deemed himself bound by the Department’s contrary decision in Freeman v. Summers, 52 L. D. 201 (1927). There, the Secretary had written:
“While at the present time there has been no considerable production of oil from shales, due to the fact that abundant quantities of oil have been produced more cheaply from wells, there is no possible doubt of its value and of the fact that it constitutes an enormously valuable resource for future use by the American people.
“It is not necessary, in order to constitute a valid discovery under the general mining laws sufficient to support an application for patent, that the mineral in its present situation can be immediately disposed of at a profit.” Id., at 206. (Emphasis added.)
The hearing examiner ruled that Freeman v. Summers compelled the conclusion that oil shale is a valuable mineral subject to appropriation under the mining laws, and he upheld the Mountain Boys and Shoup claims as valid and patentable.
The Board of Land Appeals reversed. Adopting the findings of the hearing examiner, the Board concluded that oil shale claims located prior to 1920 failed the test of value because at the time of location there did not appear “as a present fact ... a reasonable prospect of success in developing an operating mine that would yield a reasonable profit.” (Emphasis in original.) The Board recognized that this conclusion was at odds with prior departmental precedent, and [662]*662particularly with Freeman v. Summers; but it rejected that precedent as inconsistent with the general mining law and therefore unsound. The Board then considered whether its newly enunciated interpretation should be given only prospective effect. It found that respondents’ reliance on prior rulings was minimal and that the Department’s responsibility as trustee of public lands required it to correct a plainly erroneous decision.5 Accordingly, it ruled that its new interpretation applied to the Mountain Boys and Shoup claims, and that those claims were invalid.
Respondents appealed the Board’s ruling to the United States District Court for the District of Colorado. The District Court agreed with the Board that by not requiring proof of “present marketability” the decision in Freeman v. Summers had liberalized the traditional valuable mineral test. But it found that Congress in 1931 and again in 1956 had considered the patentability of oil shale and had implicitly “ratified” that liberalized rule. Alternatively, the District Court concluded that the Department was estopped now from departing from the Freeman standard which investors had “relied upon ... for the past half-century.” Shell Oil Co. v. Kleppe, 426 F. Supp. 894, 907 (1977). On these grounds, it reversed the Board’s ruling and held that the claims at issue were valid.
The Court of Appeals for the Tenth Circuit affirmed. 591 F. 2d 597 (1979). It agreed with the District Court that the “different treatment afforded all oil shale claims as to the 'valuable mineral deposit’ element of a location became a part of the general mining laws by reason of its adoption and ap[663]*663proval by both Houses of Congress” in the years after 1920. Id., at 604. And it held that the Department now must adhere to the Freeman rule. We granted certiorari because of the importance of the question to the management of the public lands. 444 U. S. 822 (1979). We affirm.
II
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Mr. Chief Justice Burger
delivered the opinion of the Court.
The general mining law of 1872, 30 U. S. C. § 22 et seq., provides that citizens may enter and explore the public domain, and search for minerals; if they discover “valuable mineral deposits,” they may obtain title to the land on which such deposits are located.1 In 1920 Congress altered this [659]*659program with the enactment of the Mineral Leasing Act. 41 Stat. 437, as amended, 30 U. S. C. § 181 et seg. The Act withdrew oil shale and several other minerals from the general mining law and provided that thereafter these minerals would be subject to disposition only through leases. A savings clause, however, preserved “valid claims existent at date of the passage of this Act and thereafter maintained in compliance with the laws under which initiated, which claims may be perfected under such laws, including discovery.” 2
The question presented is whether oil shale deposits located prior to the 1920 Act are “valuable mineral deposits” patentable under the savings clause of the Act.
I
The action involves two groups of oil shale claims located by claimants on public lands in Garfield County, Colo., prior to the enactment of the Mineral Leasing Act.3 The first group of claims, designated Mountain Boys Nos. 6 and 7, was located in 1918. In 1920, a business trust purchased the claims for $25,000, and in 1924 an application for patent was filed with [660]*660the Department of the Interior. Some 20 years later, after extended investigative and adjudicatory proceedings, the patent was rejected “without prejudice” on the ground that it was not then vigorously pursued. In 1958, Frank W. Winegar acquired the claims and filed a new patent application. In 1964, Winegar conveyed his interests in the claims to respondent Shell Oil Company.
The second group of claims, known as Harold Shoup Nos. 1-4, was located in 1917. In 1923, the claims were acquired by Karl C. Schuyler who in 1933 bequeathed them to his surviving spouse. In 1960, Mrs. Schuyler incorporated respondent D. A. Shale, Inc., and transferred title to the claims to the corporation. Three months later, the corporation filed patent applications.
In 1964, the Department issued administrative complaints alleging that the Mountain Boys claims and the Shoup claims were invalid. The complaints alleged, inter alia, that oil shale was not a “valuable mineral” prior to the enactment of the 1920 Mineral Leasing Act.
The complaints were consolidated and tried to a hearing examiner who in 1970 ruled the claims valid. The hearing examiner observed that under established case law the test for determining a “valuable mineral deposit” was whether the deposit was one justifying present expenditures with a reasonable prospect of developing a profitable mine. See United States v. Coleman, 390 U. S. 599 (1968); Castle v. Womble, 19 L. D. 455 (1894) ,4 He then reviewed the history [661]*661of oil shale operations in this country and found that every attempted operation had failed to show profitable production. On the basis of this finding and other evidence showing commercial infeasibility, the hearing examiner reasoned that “[i]f this were a case of first impression,” oil shale would fail the “valuable mineral deposit” test. However, he deemed himself bound by the Department’s contrary decision in Freeman v. Summers, 52 L. D. 201 (1927). There, the Secretary had written:
“While at the present time there has been no considerable production of oil from shales, due to the fact that abundant quantities of oil have been produced more cheaply from wells, there is no possible doubt of its value and of the fact that it constitutes an enormously valuable resource for future use by the American people.
“It is not necessary, in order to constitute a valid discovery under the general mining laws sufficient to support an application for patent, that the mineral in its present situation can be immediately disposed of at a profit.” Id., at 206. (Emphasis added.)
The hearing examiner ruled that Freeman v. Summers compelled the conclusion that oil shale is a valuable mineral subject to appropriation under the mining laws, and he upheld the Mountain Boys and Shoup claims as valid and patentable.
The Board of Land Appeals reversed. Adopting the findings of the hearing examiner, the Board concluded that oil shale claims located prior to 1920 failed the test of value because at the time of location there did not appear “as a present fact ... a reasonable prospect of success in developing an operating mine that would yield a reasonable profit.” (Emphasis in original.) The Board recognized that this conclusion was at odds with prior departmental precedent, and [662]*662particularly with Freeman v. Summers; but it rejected that precedent as inconsistent with the general mining law and therefore unsound. The Board then considered whether its newly enunciated interpretation should be given only prospective effect. It found that respondents’ reliance on prior rulings was minimal and that the Department’s responsibility as trustee of public lands required it to correct a plainly erroneous decision.5 Accordingly, it ruled that its new interpretation applied to the Mountain Boys and Shoup claims, and that those claims were invalid.
Respondents appealed the Board’s ruling to the United States District Court for the District of Colorado. The District Court agreed with the Board that by not requiring proof of “present marketability” the decision in Freeman v. Summers had liberalized the traditional valuable mineral test. But it found that Congress in 1931 and again in 1956 had considered the patentability of oil shale and had implicitly “ratified” that liberalized rule. Alternatively, the District Court concluded that the Department was estopped now from departing from the Freeman standard which investors had “relied upon ... for the past half-century.” Shell Oil Co. v. Kleppe, 426 F. Supp. 894, 907 (1977). On these grounds, it reversed the Board’s ruling and held that the claims at issue were valid.
The Court of Appeals for the Tenth Circuit affirmed. 591 F. 2d 597 (1979). It agreed with the District Court that the “different treatment afforded all oil shale claims as to the 'valuable mineral deposit’ element of a location became a part of the general mining laws by reason of its adoption and ap[663]*663proval by both Houses of Congress” in the years after 1920. Id., at 604. And it held that the Department now must adhere to the Freeman rule. We granted certiorari because of the importance of the question to the management of the public lands. 444 U. S. 822 (1979). We affirm.
II
The legislative history of the 1920 Mineral Leasing Act shows that Congress did not consider “present marketability” a prerequisite to the patentability of oil shale.6 In the extensive hearings and debates that preceded the passage of the 1920 Act, there is no intimation that Congress contemplated such a requirement; indeed, the contrary appears. During the 1919 floor debates in the House of Representatives, an amendment was proposed which would have substituted the phrase “deposits in paying quantities” for “valuable mineral.” That amendment, however, was promptly withdrawn after Mr. Sin-ott, the House floor manager, voiced his objection to the change:
“Mr. SINOTT. That language was put in with a great deal of consideration and we would not like to change from Valuable’ to ‘paying-’ There is quite a distinction. We are in line with the decisions of the courts as to what is a discovery, and I think it would be a very [664]*664dangerous matter to experiment with this language at this time.” 58 Cong. Rec. 7537 (1919) (emphasis added).
An examination of the relevant decisions at the time underscores the point. Those decisions are clear in rejecting a requirement that a miner must “demonstrate] that the vein . . . would pay all the expenses of removing, extracting, crushing, and reducing the ore, and leave a profit to the owner,” Book v. Justice Mining Co., 58 F. 106, 124 (CC Nev. 1893), and in holding that “it is enough if the vein or deposit 'has a present or prospective commercial value.’ ” Madison v. Octave Oil Co., 154 Cal. 768, 772, 99 P. 176, 178 (1908) (emphasis added). Accord, Cascaden v. Bartolis, 146 F. 739 (CA9 1906); United States v. Ohio Oil Co., 240 F. 996, 998 (Wyo. 1916); Montana Cent. R. Co. v. Migeon, 68 F. 811, 814 (CC Mont. 1895); East Tintic Consolidated Mining Co., 43 L. D. 79, 81 (1914); 2 C. Lindley, American Law Relating to Mines and Mineral Lands § 336, pp. 768-769 (3d ed. 1914). See generally Reeves, The Origin and Development of the Rules of Discovery, 8 Land & Water L. Rev. 1 (1973).
To be sure, prior to the passage of the 1920 Act, there existed considerable uncertainty as to whether oil shale was patentable.7 That uncertainty, however, related to whether oil shale was a “mineral” under the mining law, and not to its “value.” Similar doubts had arisen in the late 19th cen[665]*665tury in regard to petroleum. Indeed, in 1896 the Secretary of the Interior had held that petroleum claims were not subject to location under the mining laws, concluding that only lands “containing the more precious metals . . . gold, silver, cinnabar etc.” were open to entry. Union Oil Co., 23 L. D. 222, 227. The Secretary’s decision was short-lived. In 1897, Congress enacted the Oil Placer Act authorizing entry under the mining laws to public lands “containing petroleum or other mineral oils.” Ch. 216, 29 Stat. 526. This legislation put to rest any doubt about oil as a mineral. But because oil shale, strictly speaking, contained kerogen and not oil, see n. 3, supra, its status remained problematic. See Reidy, Do Unpatented Oil Shale Claims Exist?, 43 Denver L. J. 9, 12 (1966).
That this was the nature of the uncertainty surrounding the patentability of oil shale claims is evident from remarks made throughout the hearings and debates on the 1920 Act. In the 1918 hearings, Congressman Barnett, for example, explained:
“Mr. BARNETT. ... If the department should contend that shale lands come within the meaning of the term 'oil lands’ they must perforce, by the same argument, admit that they are placer lands within the meaning of the act of 1897.
“The Chairman. And patentable?
“Mr. BARNETT. And patentable under that act.”
Hearings, at 918.
The enactment of the 1920 Mineral Leasing Act put an end to these doubts. By withdrawing “oil shale ... in lands valuable for such minerals” from disposition under the general mining law, the Congress recognized — at least implicitly— that oil shale had been a locatable mineral. In effect, the 1920 Act did for oil shale what the 1897 Oil Placer Act had done for oil. And, as Congressman Barnett’s ready answer demonstrates, once it was settled that oil shale was a mineral [666]*666subject to location, and once a savings clause was in place preserving pre-existing claims, it was fully expected that such claims would be patentable. The fact that oil shale then had no commercial value simply was not perceived as an obstacle to that end.
Ill
Our conclusion that Congress in enacting the 1920 Mineral Leasing Act contemplated that pre-existing oil shale claims could satisfy the discovery requirement of the mining law is confirmed by actions taken in subsequent years by the Interior Department and the Congress.8
A
On May 10, 1920, less than three months after the Mineral Leasing Act became law, the Interior Department issued “Instructions” to its General Land Office authorizing that Office to begin adjudicating applications for patents for pre-1920 oil shale claims. The Instructions advised as follows:
“Oil shale having been thus recognized by the Department and by Congress as a mineral deposit and a source of petroleum . . . lands valuable on account thereof must be held to have been subject to valid location and [667]*667appropriation under the placer mining laws, to the same extent and subject to the same provisions and conditions as if valuable on account of oil or gas.” 47 L. D. 548, 551 (1920) (emphasis added).
The first such patent was issued immediately thereafter. Five years later, the Department ruled that patentability was dependent upon the “character, extent, and mode of occurrence of the oil-shale deposits.” Dennis v. Utah, 51 L. D. 229, 232 (1925). Present profitability was not mentioned as a relevant, let alone a critical, consideration.
In 1927, the Department decided Freeman v. Summers, 52 L. D. 201. The case arose out of a dispute between an oil shale claimant and an applicant for a homestead patent, and involved two distinct issues: (1) whether a finding of lean surface deposits warranted the geological inference that the claim contained rich “valuable” deposits below; and (2) whether present profitability was a prerequisite to patentability. Both issues were decided in favor of the oil shale claimant: the geological inference was deemed sound and the fact that there was “no possible doubt . . . that [oil shale] constitutes an enormously valuable resource for future use by the American people” was ruled sufficient proof of “value.” Id., at 206.
For the next 33 years, Freeman was applied without deviation.9 It was said that its application ensured that “valid rights [would] be protected and permitted to be perfected.” Secretary of Interior Ann. Rep. 30 (1927). In all, 523 patents for 2,326 claims covering 349,088 acres were issued under the Freeman rule. This administrative practice, begun immediately upon the passage of the 1920 Act, “has peculiar weight [because] it involves a contemporaneous con[668]*668struction of [the] statute by the men charged with the responsibility of setting its machinery in motion," Norwegian Nitrogen Products Co. v. United States, 288 U. S. 294, 315 (1933). Accord, e. g., United States v. National Assn. of Securities Dealers, 422 U. S. 694, 719 (1975); Udall v. Tallman, 380 U. S. 1, 16 (1965). It provides strong support for the conclusion that Congress did not intend to impose a present marketability requirement on oil shale claims.
B
In 1930 and 1931, congressional committees revisited the 1920 Mineral Leasing Act and re-examined the patentability of oil shale claims. Congressional interest in the subject was sparked in large measure by a series of newspaper articles charging that oil shale lands had been “improvidently, erroneously, and unlawfully, if not corruptly, transferred to individuals and private corporations.” 74 Cong. Rec. 1079 (1930) (S. Res. 379). The articles were based upon accusations leveled at the Interior Department by Ralph S. Kelly, then the General Land Office Division Inspector in Denver. Kelly’s criticism centered on the Freeman v. Summers decision. Fearing another “Teapot Dome” scandal, the Senate authorized the Committee on Public Lands to “inquire into ... the alienation of oil shale lands.”
The Senate Committee held seven days of hearings focusing almost exclusively on “the so-called Freeman-Summers case.” Hearings on S. Res. 379 before the Senate Committee on Public Lands and Surveys, 71st Cong., 3d Sess., 2 (1931). At the outset of the hearings, the Committee was advised by E. C. Finney, Solicitor, Department of the Interior, that 124 oil shale patents had been issued covering 175,000 acres of land and that 63 more patent applications were pending. Finney’s statement prompted this interchange:
“Senator PITTMAN: Well, were the shales on those patented lands of commercial value?
[669]*669“Mr. FINNEY: If you mean by that whether they could have been mined and disposed of at a profit at the time of the patent, or now, the answer is no.
“Senator PITTMAN: So the Government has disposed of 175,000 acres in patents on lands which in your opinion there was no valid claim to in the locator?
“Mr. FINNEY: No; that was not my opinion. I have never held in the world, that I know of, that you had to have an actual commercial discovery of any commodity that you could take out and market at a profit. On the contrary, the department has held that that is not the case. . . .” Id., at 25 (emphasis added).
Later in the hearings Senator Walsh expressed his understanding of the impact of the Freeman decision:
“Senator WALSH: [It means] . . . that the prospector having found at the surface the layer containing any quantity of mineral, that is of oil-bearing shale or kero-gen, that that would be a discovery in view of the beds down below of richer character.
“Mr. FINNEY: In this formation, yes sir; that is correct.” Id., at 138.
See also id., at 22-23, 26, 163. The Senate Committee did not produce a report. But one month after the hearings were completed, Senator Nye, the Chairman of the Committee, wrote the Secretary of the Interior that he had “ ‘conferred with Senator Walsh and beg[ged] to advise that there is no reason why your Department should not proceed to final disposition of the pending application for patents to oil shale lands in conformity with the law.’ ” App. 103. The patenting of oil shale lands under the standards enunciated in Freeman was at once resumed.
At virtually the same time, the House of Representatives commenced its own investigation into problems relating to [670]*670oil shale patents. The House Committee, however, focused primarily on the question of assessment work — whether an oil shale claimant was required to perform $100 work per year or forfeit his claim — and not on discovery. But the impact of the Freeman rule was not lost on the Committee:
“Mr. SWING. In furtherance of the policy of conservation, Mr. Secretary, in view of the fact that there has not been discovered, as I understand it, any practical economical method of extracting oil from the shale in competition with oil wells . . . would it not be proper public policy to withdraw all shale lands from private acquisition, since we are compelled to recognize, perforce, economic and fiscal conditions, that no one is going to make any beneficial use of the oil shale in the immediate future, but is simply putting it in cold storage as a speculative proposition?
“Secretary WILBUR: As a matter of conservation, what you say is true, but what we have to meet here is the fact that in the leasing act there was a clause to the effect that valid existing claims were not included, and so we are dealing with claims that are thought to be valid, and the question—
“Mr. SWING (interposing). I realize that, and I understand the feeling of Congress, and I think generally the country, that in drawing the law we do not want to cut the ground from under the person who has initiated a right.” Consolidated Hearings on Applications for Patent on Oil Shale Lands before the House Committee on the Public Lands, 71st Cong., 3d Sess., 100 (1931).10
[671]*671Congressman Swing’s statement of the “feeling of Congress” comports with our reading of the 1920 statute and of congressional intent. To hold now that Freeman was wrongly-decided would be wholly inconsistent with that intent. Moreover, it would require us to conclude that the Congress in 1930-1931 closed its eyes to a major perversion of the mining laws. We reject any such conclusion.
C
In 1956 Congress again turned its attention to the patent-ability of oil shale. That year it amended the mining laws by eliminating the requirement that locators must obtain and convey to the United States existing homestead surface-land patents in order to qualify for a mining patent on minerals withdrawn under the 1920 Mineral Leasing Act. See Pub. L. 743, 70 Stat. 592. Where a surface owner refused to cooperate with the mining claimant and sell his estate, this requirement prevented the mining claimant from patenting his claim. See James W. Bell, 52 L. D. 197 (1927). In hearings on the amendment, it was emphasized that oil shale claimants would be principal beneficiaries of the amendment:
“Mr. ASPINALL. This [bill] does not have to do with any other minerals except the leaseable minerals to which no one can get a patent since 1920. ... As far as I know, there are only just a few cases that are involved, and most of those cases are in the oil shale lands of eastern Utah and western Colorado. That is all this bill refers to.” Hearings on H. R. 6501 before the House Committee on Interior and Insular Affairs 3-4 (1956).
See also Hearings on H. R. 6501 before the Subcommittee on Mines and Mining of the House Committee on Interior and [672]*672Insular Affairs 4, 13-14, 16 (1956). The Reports of both Houses also evince a clear understanding that oil shale claimants stood to gain by the amendment:
“Under the Department of the Interior decision in the case of James W. Bell . . . the owner of a valid mining claim located before February 25, 1920, on lands covered by the 1914 act, in order to obtain a patent to the minerals, is required to acquire the outstanding interest of the surface owner and thereafter to execute a deed of recon-veyance to the United States. . . . From 1946 to 1955, inclusive, 71 mining claims, including 67 oil shale claims, were issued under this procedure. The committee is informed that in a few cases mining claimants have been unable to obtain the cooperation of the owners of the surface estate and have been prevented thereby from obtaining patent to the mineral estate.” S. Rep. No. 2524, 84th Cong, 2d Sess., 2 (1956); H. R. Rep. No. 2198, 84th Cong., 2d Sess., 2 (1956) (emphasis added).
The bill was enacted into law without floor debate. Were we to hold today that oil shale is a nonvaluable mineral we would virtually nullify this 1956 action of Congress.
IV
The position of the Government in this case is not without a certain irony. Its challenge to respondents’ pre-1920 oil shale claims as a “nonvaluable” comes at a time when the value of such claims has increased sharply as the Nation searches for alternative energy sources to meet its pressing needs. If the Government were to succeed in invalidating old claims and in leasing the lands at public auction, the Treasury, no doubt, would be substantially enriched. However, the history of the 1920 Mineral Leasing Act and developments subsequent to that Act persuade us that the Government cannot achieve that end by imposing a present marketability [673]*673requirement on oil shale claims.11 We conclude that the original position of the Department of the Interior, enunciated in the 1920 Instructions and in Freeman v. Summers, is the correct view of the Mineral Leasing Act as it applies to the patentability of those claims.12
The judgment of the Court of Appeals is
Affirmed.